You should keep XCode 3 around, regardless. Both versions can co-exist, so install XCode 4 into, say, /Developer4 instead of /Developer.

And when you open your project in XCode 4 it won't trash your XCode 3 project settings or prevent you from going back to XCode 3. However, any project settings you change in XCode 4 won't be visible in XCode 3.

Since my only iOS project is a single-developer one, I plan on refactoring into Xcode 4, since it lets me rethink how I structure the app. I never did a Universal iOS build of the app that I work on because I got frustrated with making it work in Xcode 3 -- it's worlds easier in 4 (if there is a better word for worlds there, use that one instead. you have no idea how much better).

I simply cannot sing the praises of how much sense Workspaces make, especially when dealing with a Universal iOS app. As a recovering Visual Studio user, they were the feature I was genuinely looking for in Xcode.

You might want to take that approach...Xcode 4 in developer preview seemed to misbehave if I imported a 3.2 project, but that could be because it was developer preview.

Finish your app.

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When you're sipping on Pina Coladas at your mansion in the Bahamas, then you can change your toolchain. :D