If your looking at Migrating a Windows SBS 2003 server to an SBS 2008 box I would definately recommend a read of David Overton's new Book "Small Business Server 2008, Installing, Migrating and Configuring" I have a copy and was also able to attend a training course delivered by David last weekend through Amitpro. SBS2008 and migrating to SBS2008 seems like a heavy handed, daunting task but looking at it after going through the migration techniques and following the steps (which is all it takes to be fair) makes the whole process look and feel so much easier.
Although without a doubt I'm pretty sure when it wants to go wrong, it will and I'm sure that not every migration will I walk out with a big grin on my face however knowing the steps to take and having a feel for how the process works is without a doubt an excellent idea.
If you're interested in reviewing or purchasing David Overton's book. Click Here