Did a Software Update occur between the two times?
What OS X versions are the other Mac's running?, if 10.6 did they apply the latest Software Update, and if they do does it also knock them offline?
This situation may occur if the password isn't provided in the format required by the network.
If you have a WEP encrpyted network, be warned newer that Mac's are not connecting to this as WEP encrpyted is insecure, so Apple is doing you a favor by not connecting to something that you believe is secure and isn't.
It could be that in the latest 10.6.8 update, Apple disabled WEP encrpytion intentionally to force the matter and you to better secure your network. (also to make WEP Encrypted obsolete as it's worthless)
To properly secure your network, read this User Tip of mine.
Also if that's not the problem then look into these other possible issues.
Lastly I can only think that someone is playing a game on you, using special software, they can "sniff" your computers MAC Address of your wireless card and then use that on their machines to prevent you from getting online.
Since the first similar MAC address is the one allowed on the local WIFI, yours is shut out and cannot connect if your second.
It's a dirty trick, but it's a possiblility.
This is the same information outlined in the earlier article, but.
This is what Apple has to say about this problem.
Please take note of the link in the last line.
If this does not help, call your ISP.
Message was edited by: dominic23
It's a problem with your machine or your router, because the Airport showed a explanation point on the Mac.
If it's a problem with Comcast, then it's still going to show a good connection to the router, but your web pages won't load.
To test this, get the Ethernet cable by disconnecting your router from the modem.
Connect the Mac to the modem directly, then pull the modem power and wait 30 seconds, plug it back in.
You'll connect to the ISP directly and your Mac should work fine, if so then it's a router problem and not your Mac.
Reset your router in it's entirely, read it's manual and set up all new passwords and encrpytion, this will knock anyone else off that's playing games with you also. (if they are cloning your MAC Address for instance.)
Read how to properly secure your Wifi here and be careful about who you give the password too.