huzzah! I made a screencast showing the activities stuff in 4.5 :)
if the embedded video causes trouble you can also get it straight from blip.tv
If you’re not going to watch the video, there’s one important thing in there: beta 1 has some bugs, so don’t try to switch activities with anything besides the new activity manager. Avoid the mouse plugin, the activity bar plasmoid, and the next/previous keyboard shortcuts – I’ll have them fixed in beta 2.
And because I know someone will forget and end up googling this: the default global keyboard shortcut for the activity manager is meta-q (meta is usually referred to as the windows key – I probably should have called it that in the video). That’s also accessible through plasma’s shortcuts config dialog, so you don’t actually have to google it at all. :)