I'm running the unstable version so I have it unmasked. When Paludis says a package is masked by eapi (unknown), it's a broken ebuild. If you're interested, you can read about it here:
http://paludis.pioto.org/faq.html#unmaskPaludis is more strict than portage.
http://paludis.pioto.org/faq.html#stricterEbuilds that are "broken" will sometimes install with portage, but Paludis won't let them even try. That's part of what I like about Paludis as you don't end up with some of the cruft that gets left around with Portage. I have actually installed the horde-framework-kolab ebuild with portage, but Paludis will still stop and go no further because of an issue with this particular ebuild.
The problem seems to me to be with the slot name going by this:
Caught exception 'Name '' is not a valid slot name' (paludis::SlotNameError) when generating cache for 'dev-php/horde-framework-kolab-20070601::kolab2'
Thanks,
Jason