Prompted by a conversation I had early yesterday, here are two companies that you may not expect to tweet in Spanish, but do (I just realized that this is a very American, ahem...North American/U.S. standpoint):
Engadget @es_engadget
Gizmodo @gismodo_ES
A questions here is who are they targeting? From the "ES" in both, it would be Spanish users from Spain.
It would be interesting to see how much traffic they capture from the U.S. and Latin America. I'm reminded of Joe Kutchera's book that discussed this in the Global or Local part of his book.