In the mood to live dangerously?
The Hackint0sh forums have posted a method for enabling MMS on your second-gen iPhone, while The BeatMix (with a little help from the Dev-Team) have a recipe for tweaking iPhone 3G/3GS OS 3.1.2 to enable tethering.
Rank amateurs should stop reading right about here: both exploits require not only a jailbroken iPhone, but a solid understanding of the OS internals. However, the general consensus on the forums is that both hacks will eventually wind up as easy-to-install mods via Cydia.
For those who can't wait: roll up your sleeves, dive right in, and tell us how you do!