Multiple wireless networks on a DD-WRT router

One of my clients was experiencing some odd wireless dropouts on the computer set up the farthest from the router.  They mentioned that it started around when they gave their wireless key to a neighbor so he could get online.  It seems like a neighborly thing to do, until you consider that this new computer that they have no control over is on the same network as all of their machines, as well as their file server.  I told them I was uncomfortable with that setup, but if they wanted to share their access, I would find a way to make it work.

They have a WRT54G v8 that I had them purchase from NewEgg, on which I decided to swap out the stock firmware with the open-source DD-WRT firmware.  There's a great walk-through on that covers the additional VxWorks Killer step necessary for the v7 & 8 models.  The DD-WRT firmware gives you near-complete control, especially over things like wireless transmit power (the stock setting is between 20-40mw) which I leave set to DD-WRT's default setting of 70mw.  After setting up the wireless network and the security, I went back downstairs to find that the signal was now solidly stable.

But what about their neighbors' access?  Using the extremely well-written guide available from Pennock's World about setting up multiple BSSIDs using DD-WRT, I was able to get multiple wireless networks running off of one router.  The rules in the firewall section stop any traffic going between the two networks, while still allowing the second network access to the internet.  They can even be set with their own security schemes, so the neighbors can use any old WEP-enabled device while all of the important computers connect via WPA2.