In the previous tablets there was ogg-support, but just to play given files. The metadata-crawler cannot read ogg tags, though, so there is no easy way to access your ogg files from the mediplayer. No sorting by album, artist, etc. (You can hack the DB though.)
Dissatisfied users asked for the opening of the code of the player and the crawler so we could really integrate ogg into the system, but it won´t happen for whatever reason. What I heard is that for Fremantle Nokia created a totally new and more open player and a metadata DB system, even based in a popular FLOSS program I am not familiar with. I hope ogg users won´t face the same problem in the N900, then...
As for being official or not, I'm not personally concerned. As I understand, the freedom fight stops when users are allowed to fix the system. We can't ask Nokia to be ogg enthusiasts and push it to the uninformed users.
One other concern that exists is whether the ogg codec has access to the same resources the other official ones have. If we can't make an ogg decoder use some specialized and closed DSP chip while the wma decoder can, that's reason to get angry. I'm not sure if that happens in the N900. I remember a discussion once where a Nokia person said that while that was true, the implementation of a non-official ogg decoder could still be made better than the official, closed and privileged alternatives... (Sorry for the excess of uncertain statements. :] )