I've just installed the release version of 5.1 and am getting a showstopper which is going to cause me to remove and revert back to 5.0.
Steps to reproduce:
1) Rename a folder
2) Open up one of the files in the folder to rename the namespace
3) Alt+Enter on the namespace and choose to move to namespace to match new folder structure. Something happens and it looks like the file and any referencing files are checked out but when I look at the Souce Control Explorer the scc status icon suggests that it is not checkout out. The namespace is NOT changed to match the new folder structure and I cannot edit the code using the keyboard, it has become read-only?
4) If I click the refresh button on the Source Control Explorer then the file icon changes to show that it is checked out.
5) The code becomes editable and the namespace change functionality works again.
Another possible side-effect I noticed is that the new namespace is not visible from other referencing projects.