Could be a simple matter of protocol incompatibility, as there are a LOT of non-Apple routers on the market that Airport cards simply will not negotiate a connection with. Basically unless it's an Airport base station you are using, it's hit and miss as to whether the router will support an 11b Airport connection. I think Airport Extreme is a tad more compatible, but an iMac I dont beleive has any way of supporting this. There is also the possibility that the Airport card is simply on the way out, as they are failing quite a bit these days.

Personally, I think your best bet would be to find a Mac-supported external USB WiFi dongle to connect with, as it will likely give you a lot less headaches. Unfortunately, I couldn't tell you which models may be compatible as I simply went to a wired connection when Airport became too hard.