Imported all my address entries from Thunderbird and sync’ed with me iCloud account. Unfortunatly none of the birthday entries are now shown on my Apple devices (Mac with Lion, Ipad 3, Iphone 4s).
The same happens for new entries created in eM: they miss the birthday information when showing on the Mac side. The other way around (entering a birthday under MacOS / iOS) does work - shown correctly in eM.