I seem to recall from the Nokia show that next Maemo device would be release in the second half of 2010. At a guess I think Oct 2010 would be the earliest.

As to what you would gain, I really don't know. I haven't been involved with Maemo to the extent that other have been. But I'll take a Scientific Wild Assinine Guess at it. All of your personal information (contacts, call history, perferences, etc) are stored in the same manner as they would be on a desktop PC. Upgrading to a new Linux version on a PC is generally seamless. Backup my data, boot up with the new version and run the installer, reboot and it's done. When moving to a new PC, it's simple as well. Install Linux, copy over my home directory to the new system, login and I'm done.

Similarly, moving from a N900 to Maemo 6 device should be fairly seamless. Create a Backup on the N900 (This will get all of your personal information), backup the rest of the information from the MyDocs partition, copy the N900 Backup files to the Maemo 6 device, Restore from the N900 Backup file, then let the Maemo 6 device restore your applications from the Backup. restore all of the MyDocs information, take out the SIM card and microSD card and put them in the Maemo 6 device. And you should be good to go with the new phone.

As I previously said, the above is all pure speculation. I know the N900 is right for me. But I think the question you should ask yourself "Is the N900 the device for me NOW?". You will have to put up with it for nearly a year. It is an immature smartphone and we don't have any idea how fair Nokia is going to develop the N900's feature. If you do get a N900, I don't think you'll be wasting your time in regards to getting your information on it, configuring it to your tastes, etc.