I had an interesting problem with a PowerBook G4 connected to a Verizon DSL connection. The Verizon DSL line had a Westell 327W (specifically the A90-327W15-06 model) gateway, which appears to be a combination DSL router, switch, and wireless access point. The PowerBook, which was running Mac OS X Leopard 10.5.0 and then 10.5.1, would not connect. The network was an open one, so neither WEP nor WPA were involved.
Meanwhile, a MacBook Pro sitting next to this computer could connect the first time, every time.
Instead of a successful connection on the PowerBook, I got "connection timeout" errors and the following in /var/log/system.log (which you can view with the Console utility):
Nov 22 07:42:13 localhost airportd[120]: Error: Apple80211Associate() failed -6
Nov 22 07:42:13 localhost SystemUIServer[138]: Error: airportd MIG failed = -6 ((null)) (port = 65811)
Nov 22 07:43:42 localhost airportd[230]: Error: Apple80211Associate() failed -6
Nov 22 07:43:42 localhost System Preferences[206]: Error: airportd MIG failed = -6 ((null)) (port = 53899)
The PowerBook would not associate with the access point. Using the /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport -I command line tool didn't show an associated network.
I found, after some Google sleuthing on a working computer, that there was a firmware update for the Westell gateway. In my experience, if you have problems with a consumer grade router or gateway, one of the first steps you should always take is checking on the firmware version of the device and updating it if there's a newer version available.
You have to download the firmware update for this particular Westell model from Verizon. I found that the downloadable Windows installer didn't work in this situation; multiple attempts to run it resulted in failed upgrades. (Thankfully, the gateway still worked afterwards.)
The downloadable version for Macs (and presumably anyone else not running Windows) — which to me looked more like the more-traditional "download the firmware image file and upload it in the Web interface of the router" — worked fine. I found that one at the end of this thread at MacRumors and there's also some useful information in this thread at DSLReports. The gateway updated to version 03.02.01 successfully using the downloaded upgrade file.
I need to point out that navigating the Verizon site (or sites) was horrible and didn't get me results when seeking the firmware upgrade; only Google searches did.