In the comments to this hint and others, the protocol for an iTunes Music Store search is listed. These protocols will open iTunes and take you directly to the search results page. This works great as a search template in Launchbar 4, and I assume also in QuickSilver and Butler (I haven't tried those). You can only search with a single search term (you can't search for artist and album at the same time, for example), but you can make different templates for artist search, album search, etc., as well as a generic search.
Here's how they should look in LaunchBar:
Generic Search
itms://phobos.apple.com/WebObjects/MZSearch.woa/wa/search?term=*Advanced Searches
itms://phobos.apple.com/WebObjects/MZSearch.woa/wa/advancedSearchResults?songTerm=* itms://phobos.apple.com/WebObjects/MZSearch.woa/wa/advancedSearchResults?artistTerm=* itms://phobos.apple.com/WebObjects/MZSearch.woa/wa/advancedSearchResults?albumTerm=* itms://phobos.apple.com/WebObjects/MZSearch.woa/wa/advancedSearchResults?composerTerm=*[robg adds: I'm not sure about QuickSilver, but for Butler, just create new search engines (in the Engines section), and put everything except the * into the Address field for each search. Then go to Configuration and add a new Smart Item: Web Search, pointing to whichever search you want to use and assign it a hotkey.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040426145821220