Macbook Pro can't see Airport Express

I have an older Airport Express together with a recent iMac and MBP, both running 10.6.2. The iMac can see the Airport Express, configure it in Airport Utility, and access the internet fine. However, the MBP can't find the Airport Express at all - it doesn't even show up in Airport Utility. I've been beating my head against the wall all night trying to fix this, but no matter what I try, the MBP cannot see the AX. It can see that my network is available, but trying to login to the network results in an immediate connection failure, with a system.log entry of:

*Nov 19 21:51:49 Greywolf Apple80211Agent[461]: Error joining NETWORKNAME: Connection failed (-3930) result unavailable)*

Any ideas?