Switching context using thumbnails can be useful in some cases, such as switching between "sub-windows" of an app, like in Safari. But for global app switching, what do you think is on average easier to spot:
-The thumbnail showing the ever-changing content of an app
-The app icon, which is constant and never changes.
I would say that it depends on how long ago you used the app that you're looking for. If you used it recently you probably still has the actual content in mind, and a thumbnail certainly can help, but since you used it recently it's probably only one or two swipes away in iOS 4.3. If on the other hand you used the app a few days ago, you may probably not remember the content as well and you might be better served by an icon row or grid.
Also, thumbnails have a varying usefulness depending on the app and its current content, sometimes it's hard to differentiate between multiple thumbnails.