I wrote a small patch to hilight `which.*` and $(which.*) usage in ebuilds as errors.
Created attachment 117387 [details, diff] gentoo-syntax-bad_which.patch Here is the patch against the svn trunk.
Fixed in SVN, r33. Look for a release coming later tonight.
FIXED in the 20070506 release, which should hit you sometime soonish. Thanks for the report!