Another new version of L4D Launcher is available, updating it to version 126.96.36.199673
This minor update allows modified install locations to be automatically detected via Steam's application list.
I can assure you that the next major release will include support for Left 4 Dead 2!
Update March 3, 2011:
The project has been migrated to SourceForge, as project L4D Launcher
. Binaries can be downloaded from there, but the source code is also freely available through SVN.