It appears there's a new cache/tmp folder in Leopard - /var/folders. The subfolder names are hashed, but from this mosxhints post we learn that you can read the hash using getconf, so we can jump directly to the cache folder using:
open $(getconf DARWIN_USER_CACHE_DIR)
Looking closer, this is very similar to Tiger's /var/tmp/folders/userid except there's a lot more stuff in there under Leopard.
This seems like a good place to look if you're experiencing mysterious HDD space loss. My account's /var/folders directory is 1.55 GB in size! Better keep this place in mind for user management scripts…
du -hd 0 $(getconf DARWIN_USER_CACHE_DIR)
The ATS caches have also been moved in there which is good to know for troubleshooting font issues (although they can now be cleared using the atsutil command as well).
I ran into this folder trying to figure out where my broken video capture application (icecap) was putting all the media - seems this is the default temp location for QTKit as well…