I'm currently in the middle of a migration from a SBS 2003 environment, to a 2008 environment, with separated Exchange server.
This thread is nothing more than me ranting about it. It's not very pretty to say the least. From the extremely slow NTBackup / Restore (user shares and profiles) to having the SBS box continue to try and dominate the playing field even after grabbing the FSMO roles on the 2008 DC.
Exchange 2k7's been extremely painful (and yet to be working for me), but right out of the box Exchange (pre SP1) isn't supported on Server 2008 should say something about it.
THANKFULLY - there's places like this site, Experts-Exchange, and Technet to hit up the tortuous bits.
But boooo to MS for not having better support for their own products. It's a shame that the sites mentioned above are the backbone of anybody trying to accomplish a migration on their own products. **(I know Technet is theirs, but it seems like a 3rd party solution)**