Not so classy IMO to announce it at a Press Conference! YUCK
I would usually agree that celebrities who use charity to boost their profile or to seek better PR for themselves are awful (it's usually z-listers or "personalities" who do this) but I think when it's someone like David Beckham the gesture is from the heart, and maybe by going public about it he's hoping to inspire others to follow suit?
I just can't think badly of someone of Beckham's status doing something like this. It's a brilliant thing to do and I think some are far too cynical. Like I said, I know there are celebs/zelebs who use charity for their own ends and I hate that too but I get no vibes whatsoever that this is what is happening here.