I've had this problem for a few months on both my desktop and laptop. I searched around and found that IE9 and/or desktop gadgets might be the culprit. I have narrowed it down to a specific gadget for me (at least on my desktop computer): "System Monitor"
by Incomps Creations http://www.buildagadget.com/
I found by a process of elimination that by disabling this one gadget, Alt-Tab behavior returned to normal. Of course it's possible that other gadgets might cause this problem too, and that it might depend on other things that I have on my system, like
IE9.
There's also another odd behavior since a recent update--some of my gadgets have a conspicuous white border around them. go here to see the screenshot: http://goo.gl/WirCq
Edit: it seems like it's not just that one gadget. When I wrote this post originally that was in fact what was happening. But since then, probably following the time I rebooted, this weird alt-tab effect happens when I have any gadget open. Once I close
all of my gadgets, the effect goes away. MS needs to patch this problem!