Pretty sure this has appeared in OS X 10.6.2. Basically, pressing CMD+TAB or the 4 finger swipes will bring up the switch applications bar, and I'll switch apps. Say I'm flicking between iTunes and Safari, sometimes Safari will just disappear from the switch applications list and I end up selecting Finder instead. Other times, the switch applications list just doesn't appear, but it still switches applications (i.e. it doesn't appear but if I press CMD+TAB twice, it'll switch to whatever application is third in the list). I use CMD+TAB (4 finger swipes) all the time to switch between two active applications. If I'm working in Final Cut, I'll switch between FCP and a PDF file that has the sequence I'm working on outlined. Now, when I flick between the two, I end up switching to the wrong application etc. This only happens if you are switching quickly. Press CMD+TAB and then press it again a couple of seconds later and you'll experience it too. It is really irritating and affects my work flow!