I don't have the document available right now, but I thought I read in the release notes that the 3.0 SDK could now to used to submit 2.X apps (indicating it's still fine). Also, on iTunes connect there is a check-box for "Compiled with 3.0" again indicating it doesn't need to be.
Black Wolf, can you point out some documentation or something to show that? This would be very major.
(Edit) From the iPhone developer page: "iPhone SDK 3.0 GM Seed can now be used for submitting iPhone OS 2.2.1 and iPhone OS 3.0 applications to the App Store."