Full disclosure: I haven’t tried this (yet) and technically, it could be against the AT&T TOS. Not to mention you’re going to install something to your iPhone, which could hose it….generally a reset through iTunes should fix it but no whining to me if you try this and all the Apples fall from your tree, er phone.
Having said that, let’s get to the fun part: looks like you can tether that iPhone for modem use! You’ll be limited to the meager EDGE connection of course, but if you follow these instructions you should be tethered to a notebook and on the web. Essentially, the approach installs a SOCKS proxy server on your iPhone and you then use WiFi to connect your device and iPhone through an ad-hoc network connection. It’s a true wireless tether in that respect, similar to a Bluetooth approach. There’s quite a bit of work involved to get to this point, but if you really want to open up your iPhone (figuratively, not literally), this ought to do it.