The team has worked incredibly hard to ensure that every V1.1 application works great on Whidbey, but there are a handful of issues mostly related to security and correctness that we felt like we had to fix. Kit just got this list published. We also published the list of members that are obsoleted in Whidbey beta1. We would love to have your feedback on how any of these impact your applications.
For reference, other breaking change information can be found here: http://www.gotdotnet.com/team/changeinfo/default.aspx