The problem:
Existing network based on Linksys WRT54GS router running Tomato version 1.28. Everything working fine with wireless connections from IBM Thinkpad, Dell desktop, Windows mobile 6.5 phone, Wii, Sony PSP, etc. etc. EVERYTHING can connect.
Brand new Dell XPS L701X arives and will not connect. Laptop can see router; router can see laptop; will not connect. Much time spent both by myself and Dell support engineers, and we finally pinned it on the router by being able to connect to a different router.
The Solution:
Found the following link: http://www.sevenforums.com/network-sharing/54431-windows-7-laptop-wont-connect-wireless-w-tomato.html .
Well done to Roger for working this out on his own - persistence and probably a bit of luck bears fruit.
The problem is that there is some sort of incompatibility with the "Afterburner" option in Tomato (Advanced - Wireless). Disable it and everything works fine.
Hope this helps someone as it did me.
N.