Does Xcode 4 run "stable" on OS X 10.7 DP? and will Apple accept software built in this environment for iTunes AppStore and Mac AppStore?
If you're thinking about using Xcode 4 and Lion as a "stable" coding environment, don't do it now. Lion has a lot of bugs already "enjoyable" and quite possibly a lot more to be found in the following weeks. You'll have to reinstall your OS/Xcode quite often when new releases become available. 

It's nice to try things ("new" AppKit stuff) and investigate if certain things work in Lion or how will they react to new tech like "Versions", Full screen, apps being resizable from any corner, etc. All these new things will require new code and current devs will certainly want to add these capabilities. That's what this is all about. 

Other OS X betas in the past were certainly stable (especially after a couple of releases) but I can't recall a "first" one being barely usable beyond experimentation. 

Launchbar is buggy as hell at the moment.