I recently ordered the Linksys WRT54G v5 because I heard that there are some incredible tools out there to play around with this router, specifically loading linux and a new web console to this device. However, when it arrived and I started doing some research it turned out that without tearing the thing open, and using a JTAG connector, you were stuck with it's out of the box settings.
So after letting it sit for a while (until my other wireless router started acting flaky) I decided to see how far the community had come. It turned out that the day before I decided to check back, someone had cracked it and they now a load.
In about an hour of time (which I spent most of it watching Miranda play), I was able to reflash it to the new software. I am definitely impressed by the options that are available on this router with the new flash. Out of the box, I was barely able to make a connection on the far side of the house, but now am able to set how powerful a signal it has (be careful) and can get 4 bars.
Now if you are thinking about doing the same, I would recommend finding a version 4 wrt54g, or paying more to the wrt54gl (L is for linux). Using the micro version of the flash for the 54g (because it has half the memory), leaves out a number of cool features. But for the price and power and toy factor, this is an endeavor I highly recommend for the geek at heart.