iMac not connecting to wireless network

Additional Info: customer just emailed me giving me more information on the iMac.
It says two things. It can't join the network. And the server cannot be contacted on port 110.  It says the computer is not connected to the Internet.

Presumably only one of the AP's is acting as a DHCP server?

Does the Mac connect to the router, but not obtain an IP address by DCHP?

You could try setting the Mac to a fixed IP. Pull down the Apple Menu to System Preferences. This is the Mac equivalent of Windows Control Panels. Go to the network preferences pane and select the Airport connection. Then go to the TCP/IP tab and change it from DHCP to manual. Set the IP address to an address on the same subnet as the AP and the Router (which should have different IP's by the way). Set the subnet mask to 255.255.255.0. The router address should be the IP address of whichever router is connected directly to the internet. Use the router address for the Domain Name Server address.

The setup is as follows:
A master wired router which handles DHCP, 192.168.1.1
2 other routers setup as AP's, no DHCP, One on 192.168.1.253 other on 192.168.1.254
Both AP's set to same SSID
Each AP set to a different Wireless Channel
As I mentioned before all the other Wireless PC's work fine and I can roam around the entire house.
Mac Worked fine until I added the second AP

I did not know if there is some strange Mac thing, simular to I can never seem to get a Mac to work on networks that have WEP 64. Yet I change to 128 or WPA and I am fine.

Thanks. Does the Mac see the SSID? Does it show as being connected? What IP address is shown in the Network Control Panel?

I am trying to figure out if you are having trouble connecting wirelessly or whether the problem is that you are connecting, but are not picking up a IP from the DHCP server.

You might also check to make sure the firmware is uptodate on your router and AP's.

Are these Linksys devices?

I will not know until I go there tomorrow and check the settings.  The Hardwired router is a linksys, the other two (set as AP) are a linksys and a belkin.

Well, I went to my customers home to check out the problems.  The iMac could see and connect to the wireless network just fine but just could not get an internet conneciton.  I performed all the standard reboot everything and still no internet.  I tried to let the iMac diagnose the problem and it could not solve the problem.  I tried setting a static IP, but still no Internet.  Disabled and enabled the Airport in the Mac.  Disconnected from the network and reconnected, rebooted.  All other computers in the house had no problem, including an iBook.  I finally resolved the problem by changing the SSID name of the network. It found the new network and connected fine.

Weird!

Well, I am not out of the woods.  Customer called today and he shut down his iMac last night and this morning it will not connect to the internet again.  I don't know if it would shed some light on the subject but he upgraded the OS on this iMac 2 weeks ago.

it seems as if the imac is trying to connect to the old network i would try removing all the networks in the preferred network list on the imac (apple system preferences then network and then select under show airport and then you can either set it to only connect to one or connect threw a list - in the list remove all the preferred and if you want to simplify it make it only connect to the one network)

Hum, I will go back today and check, but he said even when he manually selects the correct network name it connects but no internet.  He also said that if he manually selects the correct network, I keeps asking for a WEP code.  NONE was assigned.

If he left the default SSID on his router, the Mac may be trying to connect to another network with the same SSID. Suggest he change the SSID on his router and try again. If the Mac is asking for a WEP code, then whatever it is trying to connect to has WEP enabled.

No, I changed the SSID yesterday from the default "linksys" to linksysSM" the iMac then connected fine. But when it rebooted this morning, same thing, connected to the network but no Internet Connection.  All 4 other computers both hard wired and wireless have no problems including an iBook.  Could it have anything to do with upgrading to the newest OS from MAC?

Yes, this was Leopard.  Well, it is too late to try your fix (Strung).  I have no idea what is going on but, I seemed to have fixed the problem by replacing the AP that was closest to the iMac.  Originally I had an old Linksys Wireless router (set as an AP) and a newer Belkin Wireless Router (Set as an AP).  I replaced the older Linksys router with another Belkin unit and everything seems to be just fine now.  But it has only been a  day or two now, so maybe it will crap out again.  Thanks for all  the help and suggestions,  I will keep this question open for a little longer to make sure I have actually sloved the problem.