thunderbird-lightning doesn't install with rawhide, due to the newer thunderbird package (thunderbird-3.0-3.7.b3.fc12):

[spot@pterodactyl ~]$ sudo yum install thunderbird-lightning
Loaded plugins: refresh-packagekit
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package thunderbird-lightning.x86_64 0:1.0-0.8.20090513hg.fc12 set to be updated
--> Processing Dependency: thunderbird < 3.0-3.6.b4 for package: thunderbird-lightning-1.0-0.8.20090513hg.fc12.x86_64
--> Finished Dependency Resolution
thunderbird-lightning-1.0-0.8.20090513hg.fc12.x86_64 from rawhide has depsolving problems
  --> Missing Dependency: thunderbird < 3.0-3.6.b4 is needed by package thunderbird-lightning-1.0-0.8.20090513hg.fc12.x86_64 (rawhide)
Error: Missing Dependency: thunderbird < 3.0-3.6.b4 is needed by package thunderbird-lightning-1.0-0.8.20090513hg.fc12.x86_64 (rawhide)
 You could try using --skip-broken to work around the problem
 You could try running: package-cleanup --problems
                        package-cleanup --dupes
                        rpm -Va --nofiles --nodigest

It probably just needs to be rebuilt and have the hardcoded Requires adjusted.