So, in between work, playing WoW (actually very little, these last few weeks), playing MechCommander 2, and watching anime (Bokurano, Lucky Star, D.Gray-Man, and Death Note), I've been researching Japanese adjectives and verbs a bit. You could call this an errata post on the topic.
Hmm, where to start? Well, first of all, I was a little bit off about the difference between the verb of a sentence and the verb of a relative clauses being identical. I was right in that there isn't a difference between the forms in modern spoken Japanese. However, according to An Historical Grammar of Japanese (don't ask me why it's 'an'; I don't know), the forms of verbs and adjectives in those two roles used to differ for some verbs (there are several conjugation patterns formed by progressive breakdown of the original grammar) in the written form of Japanese.
Japanese is a bit different than English in that there are several forms of it, spoken and written Japanese being the ones most relevant to us. Compared to spoken Japanese, written Japanese (I'm not exactly certain what writings this includes, but I'd imagine things like text books and scholarly works) is rather archaic. By my understanding (keeping in mind that I'm hardly an expert on Japanese), written Japanese is to spoken Japanese what Old English or Middle English (somewhere between the two, to be precise) is to modern English. It's somewhat similar to spoken Japanese, but there's a lot of difference in things like grammatical suffixes (like using the -nu suffix in the written language to indicate a negative, but using the adjective 'nai' in the spoken language) - enough that it would take some effort for someone who only knew spoken Japanese to figure out what was being said in something using written Japanese. I suppose you could argue that modern English retains some archaic spellings in the written form as well (like writing 'knight', even though it's said more like 'nite'), but this generally doesn't affect grammar so much, apart from some common contractions like 'wanna' and 'dunno'.
Now, getting back to Japanese adjectives. Historically, there are five distinct base forms of Japanese verbs, two of which are of concern to us, here: the predicative, the form used for the main verb of the sentence; and the attributive/substantival, used for the verb/adjective of relative clauses and when using a verb/adjective as a noun. One thing that differs between spoken and written Japanese is that spoken Japanese has replaced the predicative form of verbs and adjectives with the attributive form.
Going back to our examples from last time (and noting that we're now into the realm of written Japanese, my knowledge of which rates just above nonexistent), "the dog is bad" would be written "inu wa warushi" (predicative form), while "bad dog"/"dog that is bad" would be "waruki inu" (attributive form; note also that 'waruki' has become 'warui' in the spoken language). Both predicative and attributive forms are the same for 'kamitsuku' in modern written Japanese, so I'll use a different verb for the example. "the dog eats" would be "inu wa taburu", and "eating dog"/"dog that is eating" would be "tabu inu" (note that the verb has changed to 'taberu' in spoken Japanese).
I've done a tiny bit of looking into the matter in Korean, and it appears there is also a difference in predicative and attributive forms there, as well. This is an interesting piece of information, as it indicates that in Korean (and in older Japanese) this predicative/attributive difference took the place of the relative pronouns in English and other Indo-European languages ('that', 'which', etc.). Of course spoken Japanese has lost this interesting trait, replacing it with analytical methods, where the position of the verb/adjective alone indicates whether it's the main verb of a sentence or the verb of a relative clause.
Moving on to a different but related topic. Unfortunately, as attractive to me as the idea of a language where verbs and adjectives are one in the same is, there is some evidence in Japanese to the contrary, and this characteristic seems to me to be something Japanese (and by correlation Korean) evolved, rather than initially possessed in the language's original form (although it's not impossible that Japanese merely lost such an initial form and later reconstructed the functionality through different means; unlikely, but not impossible).
Taking a step backwards in time, we find that there are five bases to Japanese verbs: the predicative, the attributive, the conjunctive/adverbial (used as an adverb or to express things such as "he bled and died", the imperfect (used for various compound forms), and the perfect (an action that occurred in the past which determines the present state; an example from the book is "Nara no wagie wo wasureteomoe ya" - "have I forgotten my home, Nara?"). Using the verb 'shinu' - 'die' (this is actually an irregular verb, but there is evidence that it is actually the original, true verb conjugation), the forms are 'shinu', 'shinuru', 'shini', 'shina', and 'shinure' (note that two of them appear to be compound forms formed by adding a form of 'uru' - another verb meaning 'exist').
Now, compare those forms to the conjugation of the adjective; using the adjective with the root 'taka' ('high'): 'takashi' (predicative), 'takaki' (attributive/substantival), 'takaku' (conjunctive/adverbial), and 'takakere' (perfect). We can see three things, here. First, there is no imperfect form of the adjective; second, all of the adjective forms appear to be compound forms, with various suffixes tacked on to the root; lastly, three of the four conjugations have a 'k' in their suffix.
While this does not answer the question conclusively, to me it appears that initially adjectives were true adjectives (either that or nouns), consisting merely of the root, and over time were made to resemble verbs by the addition of various suffixes. This is in agreement with the fact that, in the oldest known Japanese writings (from about 800 AD), in addition to the forms just shown, we occasionally see the use of the root alone as an adjective or a noun (this fact brought to you by the same book).