Microsoft has come come out with an Upgrade Guide for moving to VS2005
Official Summary: Avoid pitfalls in the upgrading process with these best practices for converting Web projects in Visual Studio 2005. (14 printed pages)
GridViewGirl Summary: Try it. If you get errors, fix them.
That's pretty much all the article says, and that's all you really need to know. Oh, and backup first.
"If the application you are converting is of reasonable size and has several Web projects and additional projects, such as class libraries, in a single Visual Studio solution, it is possible to encounter issues during migration. Be prepared to spend the better part of a day completing the entire process. The steps and guidance provided in this article can help an informed user to migrate most applications of medium complexity."
I just got done upgrading an app "of reasonable size", and their time estimate was about right. Most of the issues I encountered weren't covered by this article though. More on that soon...
--Marcie