I wanted to see if anyone else can reproduce this prior to attempting to create a JIRA issue.
1. Open any file (.java or .jsp for example) that exists under Subversion control in two or more branches.
2. Right click file in editor and choose Subversion > Compare with Branch...
3. Select a branch where you know the file to compare against exists.
Do you see the diff screen? I do not.
I have IDEA 8.0 install side-by-side with IDEA 7.0.3. In 7.0.3 this function version works as expected.
IDEA 8.0 Build #9013
WinXP SP2 Intel Core2 Duo 4GB RAM