Mike, I can almost guarantee you'll see a whole heap of error messages that will need to be fixed, meaning that you can't simply import to VS2008 and pretend nothing has happened. ;-) I'm told that the process varies but I've seen quite a few issues relating to xml and old web page attributes like !DOCTYPE needing attention. IMHO it's reasonable to see this as an opportunity to decide whether it is better to "fix" things to use some of the new stuff in 3.5 or just "fix" the old work so the errors go away.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1