I don't know if everyone knows this, but it wasn't documented as a shortcut in the Finder menu bar ... pressing Command-T while doing a search from the Finder's toolbar, or after finishing a search, the Finder will create a folder in the sidebar called ".search by terms in location," where "location" is where the search was run. This folder will contain the search results.
[robg adds: Actually, Command-T is documented as "Add to Sidebar" in the Finder's menu. However, I wasn't aware that you could add search results to the sidebar. This isn't Tiger's smart folders, though -- you'll only be saving the results of the search, not a command that updates each time you select the results folder. So as you add and remove files from the search location, the results folder won't change. I also can't figure out where this temporary folder lives -- you can't navigate up from it, and it's not located anywhere in the user's folder that I can see (searched in the Terminal via find . -name....). While this may not be ultra useful, it is interesting that you can save a search results folder...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050406205238874