I'm as much in favor of speaking up for our rights as the next guy. I have a hard time, though, thinking of anything good that could come from 25,000+ people signing this petition. The president is already on the record as favoring an AWB--and btw, if you weren't around in 1994, you should look up the AWB and see just exactly how it declared a rifle to be an assault weapon, in addition to the list of specific firearms.
When a bill comes up, it will be in the news, so then write your congressman and senators. Signing this petition at the White House, will just give the liberals a list of 25,000 "gun nuts". It will probably also get you on a list of potentially dangerous people. If you've already signed, watch and see if there's not a black Suburban passing your house once a week from now on!
A better idea is to just get on the whitehouse.gov email list for several pertinent topics. That will just identify you as a concerned citizen, and getting the emails gives you an idea what kind of things the president is likely to be supporting.