Ovi needs SyncML via http, which the N900 doesn't support. - You could add SyncML via http as a third party app, but Ovi is set up in a so braindead way that even this wouldn't help:

They don't ever give you access to your own login credentials for Ovi. Instead, they send a SMS which is interpreted by a Nokia phone so it can connect to Ovi. (And, best part, even then it doesn't work. Syncing against Ovi doesn't work for a lokt of S60 customers, the error is always the same and Nokia doesn't care.)

So you'd have to ask Nokia to provide the details about how this password-SMS works (which they won't provide), then add this to an existing SyncML appilication like Syncevolution... well, and then you'll run intio the same error I have on my phone.

Better wait for Nokia to fix it.