I have recently heard that the phrase のみ means "only". Does it come from another compound, like の実 or something? I don't think that's necessarily where it came from, but I would be surprised if that の was not the genitive particle. (Or conjunctive? I'm not sure what the right term is.)
Expanding on Darius's comment...
大辞林 says (1) 語源は「の身」で，「…それ自身」と強調するのが原義といわれる。 Comes from の身 (GEN. body). Original meaning is emphasis of "that thing itself".
(2) 漢文における文末助辞「耳」の訓読から生じた用法。 Comes from a native reading of the Classical Chinese word 耳
(3) 現代語では主として書き言葉に用いられ，これに相当する助詞としては，一般に「だけ」「ばかり」の語が用いられる In current usage, the word is mainly used in written material. Generally, 「だけ」 and 「ばかり」 are used to express the same meaning.
In case you don't understand (2), 耳 is used in Classical Chinese (漢文) as a contraction of 而已, a final particle meaning something like "that is all", "nothing more", "that is the end of that". The kun reading of 耳 is みみ, which was modified to のみ, according to this theory. Note that this matches up with its usage in Japanese as a final particle, in sentences such as the first comment below.