The PS3 might not be setup to look for ad-hoc signals. I don't have one so I don't know. Personally I would like to take a second to remind you that although you might have an unlimited data plan with the cellular company most policies have an implied contract that states somewhere in there that it is unlimited "within reason" meaning that your download/upload from the PS3 and xbox360 will probably quickly exceed what would be normal downloading limits for someone surfing the web from a pocketpc.
Although I'm not sure about the accuracy of this page, I saw this news story a month or two ago:
http://www.wltx.com/news/story.aspx?storyid=56628.