Hi,
well can't still believe it, but today with itunes 8.02 and outlook 2007, after reading this article:
http://support.apple.com/kb/TS2456
all birthdays are shown correct !:)
I enabled under itunes, after clearing sync history in prefernces, the overwrite all contact entries on iphone, and voila - nearly everything got their correct.
The year-days are still sifted one day ! (Maybe this get's fixed with itunes 8.03 🙂
And it's also the first time that all contacts on mobileme show the correct birthdays.... (year-days are still also sifted)