"The debug settings are not valid. Restart the SharePoint Workflow Project Setup Wizard to enter the settings again."
Thanks to this blog entry, I found what was wrong, though not why it was wrong. Somehow, the "local site [that I] want to use for debugging" (found when you right-click on the project and go to SharePoint Debug Settings) was reset to http://
I was using a SP Sequential Workflow project in VS 2008 w/ an InfoPath form, deploying to a MOSS 2007 server.