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?
Try this
This situation may occur if the password isn't provided in the format required by the network.
http://support.apple.com/kb/TS3611
Also,
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.
WiFi security issues, at home and WiFi hotspots
Also if that's not the problem then look into these other possible issues.
WiFi, Internet problems, possible solutions
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.