We're investigating a problem with just-released Unity 4.1.1 where user projects that contain precompiled assemblies (like many asset store packages), cause the project to no longer compile in MonoDevelop and VisualStudio. We are still investigating, but seriously considering reverting the change (assembly signing) that caused this. Because of this we strongly recommend asset store sellers to not yet adjust their packages to be 4.1.1 compatible, as that will render them incompatible with previous Unity versions. Since the only change in 4.1.1 (besides bug fixes) has been one that has caused this problem, and provided no user visible feature, we have temporarily pulled 4.1.1 from the website. We'll keep you up to date in this thread. Right now it is looking likely that we'll soon do a 4.1.2 release where we no longer sign UnityEngine and UnityEditor which would make this problem go away. Apologies for this inconvenience, stay tuned for news.