The RPM Fusion Free package repositories now(¹) contain the popular thunderbird extension enigmail for F9, F10, F11/rawhide and EL-5. To install it just configure RPM Fusion and run:
# yum install thunderbird-enigmail
Big "thanks!" goes to Remi Collet for packaging it up and submitting it for Review in RPM Fusion.
The package is especially helpful for users of rawhide/Fedora 11, as the enigmail homepage doesn't offer prebuild extensions for the thunderbird beta that Fedora 11 contains. It's also nice for users of Fedora for x86-64, as the add-ons on the enigmail homepage sometimes were outdated.
Additional note: Normally RPM Fusion doesn't take any packages that are suitable for Fedora to avoid getting in Fedora's way. But earlier attempts to get thunderbird-enigmail into Fedora have failed, so we took it; hopefully the package can be moved to Fedora sooner rather than later!
(¹) might take up to something like 32 hours till your yum sees it due to mirror lag and metadata caching