I though i'd post a couple of links I have found useful..

DennisAn wrote a great whitepaper on how to target different runtime versions... http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnnetdep/html/sidexsidenet.asp

And if you want to know the details of what is the breaking changes in V1.1 you can check out this list:

This list we generated from beta customers, our own testing and our own "by design" changes:
http://www.gotdotnet.com/team/changeinfo/Backwards1.0to1.1/default.aspx

Should we have a contest to see if who can find a breaking change not listed?  ;-)

Hope that helps!