Connecting two Linksys WRT54GS routers together
I agree that the hardline is the best solution, and also most easily preserves the security with minimal fuss and cost.
If you really wanted to avoid running that line, then another option would be to try a wireless bridge.
Router A -> Wireless Bridge -> hardwired to Access Point / Router B.
The Netgear WGPS606 Wireless Print Server has a wireless bridge built-in that works well in my experience as a simple bridge. It's highest security at present is WPA-TKIP (which is probably fine).
However, there's a very good chance that this won't work. If you want to pursue this further, please ask me directly to test it out. I'm concerned that the bridge might get confused by the two wireless routers, and if it decides to bridge to the 2nd one at any point, it will simply fail, and possibly cause IP conflicts in the process. MAC filtering might be able to avoid this problem.
Some AP's will do wireless range extention via WDS, however, with consumer gear at least, this often degrades the security to WEP, which IMO is too low, making this option not recommended.