I've read a number of references to problems with former projects importing into CS4. I too thought I had a big problem when I imported a project to CS4 and the workspace was blank. Once I discovered the new feature in the Workspace: "Import Workspace from Project", UNCHECKED it, and selected an existing CS4 workspace - there was no problem.
So far, all of my projects have imported just fine.
The workspace iformation is stored in an xml file named UserWorkspace(number).xml in the Documents and Settings\[username]\My Documents\Adobe\Premiere Pro\3.0\Layouts (or ArchivedLayouts) folder. Additionally the workspace should be stored in the project file itself. If you can't reopen a workspace that you've saved, it's because PPro can't find the xml and is creating a new default workspace. This can happen if you are using a roaming profile or folder redirection (Premiere Pro doesn't understand roaming profiles very well). If you log on as a local user or reset your user profile to a local profile, you should be fine.