Hacking the WRT54G
Yep. Of course I've played with hacking the wrt54g. What linux nerd hasn't?
Serial Port Hack
Here's pictures of my serial port hack. I did this to a wrt54g version 3. You'll see in the last photo, a screenshot, I've loaded openWRT on it.
Apologies for the crappiness of the photos. They were upload straight off the camera.
SD Card Hack
This was a surprisingly easy hack in all senses. The soldering was easy (save for a trouble spot or two), the case mod was easy (though I did make my slot a little too big), and the software was easy (just install some packages in OpenWRT).
I used on old, cheap, often-non-functional USB sd card reader for the SD card socket. I put a piece of blue masking tape to prevent any unwanted electrical connections being made between the card and the grounded metal of the Ethernet ports. Then I just hot-glued the slot onto the tape. It's quite sturdy.