In iTunes 10, Apple has removed the filter/search field from the Sync Apps window for an attached device. The text field allowed you to search for a string and filter your list of apps like in Spotlight. It's not really missing; it has just been replaced.
Just like in any iTunes window -- whether browsing the App Store, Music Store, etc. -- the universal Search field in the upper right can now be used to filter/search through your own apps in the Sync Apps list.
[crarko adds: I tested this, and it works as described.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100902135918278