Only a tiny number of people will be interested in this, but I recently had to open the case of a Linksys WRT150N router and found a few questions on the web asking how to do so. Doing this voids your warranty, of course.
First of all, find a T10 security torx driver. These are like standard Torx bits with a hole in the middle.
Start by flipping the router over and levering up the silver panel at the rear of the router. I'm calling the rear the side which the antennae are attached to. The rear half of this panel will pop out of the body, but the front is attached with hooks which will break if you try to lever up the front half first. Once the rear half is free, the front half will slide off easily.
This will expose four security torx screws. Remove these.
Now you can remove the top and side panels. Pull one of the side panels and its side of the top panel away from the router at the same time. The hooks which hold the side panels on should pop open one at a time without damaging anything.
Repeat this with the other side panel. You should now have a completely disassembled WRT150N, so you can get at the serial port connections you want to attach to.