Unfortunately, Cisco has now removed the 184.108.40.206 firmware link from the WRT 100 page: the only option they present is to download the 220.127.116.11 firmware, which, as I mentioned, is highly problematic. Color me unimpressed with Cisco.
For folks stopping by because they're experiencing problems with the 18.104.22.168 firmware, I don't have a lot of options. I couldn't find the copy of the 22.214.171.124 firmware, unfortunately, or I'd post it for download. My only real suggestion is to note that if one or more of your machines are Vista, some of the folks on the Cisco forums have suggested disabling IPv6 support. ("Start", right-click on "Networks", choose "Properties", "Manage Network Connections", right-click on your wireless network adapter, deselect "Internet Protocol Version 6".) I haven't tested this: YMMV: let me know if it works for anyone.
Do other folks out there still have a copy of the 126.96.36.199 version of the firmware? If so, I'm happy to throw it up on my website. I've continued poking around for open-source firmware, but none of the various options seem to support the Ralink RT2720L/RT2780 platform.
[Added 2/10/09: Other folks have reported that disabling IPv6 under Vista does in fact work for them. That doesn't exactly excuse Cisco -- nothing you send at a router should take it down, and if it does, that's really, really bad -- but it does seem like the best workaround, now that Cisco has removed the working copy of the firmware.]
[Added 2/16/09: It turns out that Linksys has an alternate version of their website, and on that alternate version, they've still got the working 188.8.131.52 firmware. In case they remove that again at some point, I've also posted a copy on my own website.]