Eclipse was working properly under Beta 1 and Beta 2, but seems to be having issues under Beta 3. The splash screen appears, and loading completes, and even shows my workspace. (Complete with projects, last-open tabs and contents, etc.)
However, the GUI thread seems completely stalled - I have the rainbow wheel of death perpetually, and cannot interact with any of the UI.
Update: I updated to Eclipse Mars (4.5) and am experiencing the same issue.