I have done about 4 migrations and I have had no problems at all when migrating from SBS 2003.

Best thing to do is use vmware converter to make a full running backup of the SBS2003 server.
When that is done you can start it on a PC with VMWare Workstation or convert it to a Hyper-V image and
then start it up within Hyper-V, now you have a test environment where you can test the migration and see
what problems come up.

Other failsave option you have is that when the actual migration fails and your physical SBS2003 server is not
working anymore and the SBS2008 is incomplete you can start up the virtual SBS2003 you have made and run it
to let your customer work on and you can start the migration from the virtual image again.

I have done it with all 4 migrations and it worked seamles without any problems.

If you need info on how I have done it just email me and I am glad to help out.

3. If you have renamed your Administrator account via Group Policy you need to disable that setting and reboot so that the default Administrator account is indeed Administrator - Sharepoint and WSUS depend on this and will fail to set up, failing the entire setup in the process.

I had issues with the migration tool. When I ran it, it would not update the schema automatically.
I had to manually run the updates

Open a command prompt and browse to the *CD*\Tools\Adprep folder

First run adprep /forestprep
Then run adprep /domainprep /gpprep
Then adprep /domainprep

After you run all three commands, and they all come back successful, you can run the tool again, and you get the succesful marks on all prep items.