so, a bug snuck into beta 2. If you’ve never switched activities, you might not *have* a current activity, which can get you weird stuff like black screens. It’s fixed in trunk (as of a few days ago), but if you’re using the betas, go press meta-tab (the global shortcut for cycling through activities) to avoid this bug :)
Also, although the beta1 activity-creation bug was fixed, it gave some people a lot of useless activities that can’t be automagically cleaned up. If you don’t want to delete them all manually, here’s how to wipe out all your plasma and activity config:
1. log out of kde
2. log into a terminal
3. remove plasma-desktop-appletsrc and activitymanagerrc from ~/.kde4/share/config/ (might be .kde/ instead of .kde4/ on some distros)
4. log back into kde
5. do the meta-tab thing again if you’re on beta 2
remember: that will delete *all* your plasma config, both desktop and panels. you might want to back up the files first, in case you change your mind.
 oh, one more note; if you’re using multiple screens or PVD, the migration from 4.4 to 4.5beta will make each containment a separate activity. so, if you had 4 virtual desktops and PVD turned on, you’ll have four activities, *each* with four containments. this might not be what you wanted. I had intended to add some code to try and create fewer activities during migration, but didn’t have time. it might still make it into 4.5; we’ll see.