Sorry, this question is old, but the answer is really wrong.
As toolbear mentioned, Doug Harris' answer is only partially correct.
The Launch Services Database file, which is where all of the information for all known applications and the types of documents they can handle is stored, is located in the following location:
Lion:
/private/var/folders/**/**/-Caches-/com.apple.LaunchServices-034501.csstore
Snow Leopard:
/private/var/folders/**/**/-Caches-/com.apple.LaunchServices-025501.csstore
Leopard:
/Library/Caches/com.apple.LaunchServices-023501.csstore
The /private/var/folders/ folder is where the "secure" Caches and Temporary folders for each user are stored. The **'s in the path shown above will be something like /dl/dlXWtS5WH9SNboPxxrDIyE+++TI/, or another obscure character sequence to prevent guessing.
Note that the name com.apple.LaunchServices-025501.csstore is composed of the user ID (mine is 501) appended to the -025 suffix, which is unique to Snow Leopard. (In each version of OS X, Apple tends to change the distinguishing suffix used to prevent compatibility problems between different versions of OS X. You'll notice that Leopard used -023). Lion is using the -034 suffix.
As far as I know, Spotlight is hardcoded to work this way. Most people in your situation simply disable spotlight, and use an alternative, like EasyFind.
If this is too drastic, you may want to clear out Spotlight's .Spotlight-V100 file at the root of your drive to force reindexing. Once reindexed, you might see better performance (and you might not).
Best Answer
The metadata is inside a hidden
.Spotlight-V100folder at the root of the indexed volume.