I know you get a 3 day headstart - but in gears 3 for example, you got special skins for the weapons if you played the beta. Are there similar rewards?
There are no tangible benefits to playing the beta. If you have pre-purchased you'll get a 3 day headstart on retail release.
There are a few benefits to playing in the beta's in that you can learn the game so that upon release you can hit the ground running with your choice of race and class.
As for actual tangible items these are linked to playing Guild Wars 1 in the Hall of Monuments. By earning points in GW1 you'll unlock items which can be used in GW2.
http://hom.guildwars2.com is used to show how many points you've accrued on your GW1 account.