On my RHEL6 system (GNOME 2), I have one or two VMs running in full-screen mode, both on the leftmost monitor of a 3-monitor setup with multiple workspaces. When I switch back to the workspace that has one or both of the VMs running, one of them will often grab keyboard focus and refuse to release it until I explicitly focus it with the mouse (which is sometimes the one in the background when both are running, making it that much more confusing). Clicking on other windows on the first workspace doesn't recover the keyboard focus, but curiously enough switching to a different workspace will recover keyboard focus, at least until I go back to the VBox workspace. It doesn't happen every time, but it's definitely often enough to notice.
I don't know if this occurred with Beta 1 (I didn't use Beta 1), but it definitely did not occur in 5.0.x.