Actually, it was done once before, and successfully. Some guy who ran a tf2 blog for the xbox got a petition of so many signatures he got sick of adding them all, then was able to rally for support on the steam forums. Alot of PC users were saying that they were willing to have their update postponed for an xbox update. So valve whipped up the settings updates, and that was that.
http://tf2xbox.blogspot.com/
We could try to replicate his past success, but we need to make it especially clear what we want, not just an update. We want x weapon or y map. And if you cannot give that to us, by god, you will tell us, instead of leaving us to wonder.
But we would need people actively trying to get signatures, and I think that the bulk of our community would have to work to get enough signatures for valve to even consider updating. It would also help if we can get someone to get the support of the PC community on our side. I think r33mix would be perfect, since he is by far the most well known member we have on the PC. But it's up to him; he'd probably suffer alot of ridicule for even advertising the petition, and I won't ask him to do that if he didn't want to.