Now that Sunbird is pmasked for removal, it would be nice to have the official replacement, the Lightning add-on to Mozilla Thunderbird, available as an ebuild. On my ~amd64 system, the add-on fails to install from the binary distribution, giving the following error message: Incompatible Extension ---------------------- "Lightning" could not be installed because it is not compatible with your Thunderbird build type (Linux_x86_64-gcc3). Please contact the author of this item about the problem. Even if the Lighning people address the issue, I'd hope that Gentoo remains a compile-from-source platform at heart, and doesn't rely on distributed binaries. Until there is an ebuild available as replacement, I've got strong feelings against the removal of Sunbird.
Find builds for not officially supported platforms at http://releases.mozilla.org/pub/mozilla.org/calendar/lightning/releases/1.0b2/contrib/
Enable the lightning useflag on thunderbird-3.1.1 and you will have it.