Came across this thread when doing some digging so I'll add my 10p.
UK (Orange) International S3 here - can connect and use WiFi OK but there are 2 slightly irritating issues.
1 - the phone will sometimes drop WiFi and takes a while to reconnect when you switch it on (resulting in the lockscreen being unresponsive and then going black instead of letting you in to use the phone)
2 - the phone sometimes takes a LONG time to scan for WiFi networks - not a problem once the ones you use most are 'remembered' but not ideal.
In both cases I'm pretty sure the issue is the WiFi going into powersave mode - which is annoying as the option to keep the WiFi alive, even when the phone is on standby, is selected.
I'm going to try a Wifi Fixer/Keep-Alive tool to see if that helps any - but clearly a better fix is required ASAP from Samsung.