I could be wrong, but I believe this is done by setting up a WDS (Wireless Distribution System). This is fairly easy to setup if you only have airports:
http://docs.info.apple.com/article.html?artnum=107454
Getting it to work with the linksys is a bit more complicated and I'm not entirely sure if it's possible. I'm not sure if the official linksys firmware supports this but I do know that the alternative firmware for that router, dd-wrt does. Here's an article explaining how to setup a WDS on a linksys router running dd-wrt.
http://www.dd-wrt.com/wiki/index.php?title=WDS_Linked_router_network
The only question is whether or not the apple and linksys would be able to work together in this sort of setup. I haven't done this setup before so I'm not sure If it'll work but hopefully it's a step in the right direction. If there's a much easier simplified way of accomplishing the same thing somebody please correct me, this was just a guess.
EDIT:
Here's an article showing how to accomplish a similar setup. It's different in that it's using the linksys as the main router and airport express to extend it. It says the only wireless security that can be used is wep, but for some reason I think I remember reading this has been changed since (I could be wrong).
Some of the steps would be similar though and I think the only changes you would have to be would be to turn off DHCP on the linksys and perhaps change the wireless access mode from AP to something else (if running dd-wrt).
http://www.downloadsquad.com/2006/1...ween-a-wireless-router-and-an-airport-expres/
Sorry if this is a bit vague, let me know if any of it helps or if you get it working, I'm interested to see if this type of setup is possible.
