sync sms to an imap folder (or directly to a database somewhere), sync mail to imap, sync contacts and calendar to a caldav or syncml (funambol) server. That's for syncing the most important logical data.

Sync raw data (MyDocs, but also configuration folders of programs you use on both device) with any file syncing solution for linux.

Issues would be no out-of-the-box clients or servers available for some, and a bit of prototypical solutions for others, but if you had a N900 and plan on preordering a N9 it won't stop you