You can use syncevolution to sync the birthdays calendar from your N900 to a google calendar. For this You should create an extra calendar in your google account. You can display this calendar with a different color.
I have not synced my birthday calendar with google, but I have synced more then one N900 calendar with google and it worked. Just see the instructions here
http://talk.maemo.org/showthread.php...1&postcount=26
and the following post.