I currently use .mac for this but it's not ideal.
I use iDisk for most of my most important files -- that way they're accessible on both computers at all times. However, iDisk is not backed up by Time Capsule, which is a problem if you accidentally delete a file on your iDisk -- it's just going to be gone.
I use the Publish/subscribe feature to keep iCals on both computers. .Mac is supposed to be able to sync calendars between multiple computers but in my experience it doesn't work very well -- you're constantly being asked which calendar entry is correct, and it's a big hassle. With publish/subscribe you can only update the calendar on the publishing computer, but it's automatically updated on the other computer(s).
For music I don't keep a complete copy of my library on both computers. I use my iMac as a music server and have a subset of my music on my iBook. But I'm not a big music listener when on the road, so you might want to keep a duplicate on your MacBook. Or you could just use your iPod for that, and simply keep a master music library on your main computer. When you're at home it's easy to listen to your music using either computer.
For email I use Gmail. On my iMac I use Mail as the client; when I'm on the road I access my Gmail account from my web browser on the other computer. I don't use Mail on that computer. But if you used .Mac mail or the Gmail iMap service, you could use Mail as your client on both computers.
