The following pairs (in one case, triples) of people on my friends lists have birthdays which coincide:
moonbeanz and loneraven, a fellow Christian and a fellow slash writer.
wag_9393 and hddod, two SF fans, both geeks.
aamanda and mrhedgehog, a psychic poet and a seller of SF.
whotheheckami and fattyc, two gentlemen of substance.
del_c and jamesb, Two SF fans.
blufive, ninneviane and beermat, a programmer, a naughty girl, and a geek.
katiemariie and sand_l, a slash writer (again!) and a teacher.
missfairchild and chazza, journalist and geologist.
purplecthulhu and peteyoung, two SF fans, both of whom look after things in the sky.
cardboard_dawn and deferenz, a fake student and an accountant.
dmw and maryread, two more SF fans.
Edit: to correct the error in mrhedgehog's description, as notified by suaveswede.
Further edit: Firstly to note that I've not listed cases where two journals are run by the same person -- there are several instances of this, the most obvious being _jamez_ and _dreamyjamez_. Secondly to note that I've not listed molesworth and long-haired fishlifter because the latter's journal is used by high-heeled fishlifter instead, and doesn't have a birthday listed.