hum, wait for BC 5.01 and ML 10.8.1 if it is anything like 10.7.0 was.
Or give Apple a few days to update their servers.
Try a restart and using Shift key and Safe Boot mode.
General Help:
http://support.apple.com/kb/HT4818
http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.7.pdf
create a Windows support software (drivers) CD or USB storage media
http://support.apple.com/kb/HT4407
Advanced Network: Proxy Settings, I uncheck "Auto Proxy detection" and make sure you have the interface you use selected and none others.
"Go into System Preferences - Network - Advanced - Proxies Uncheck "Auto Proxy Discovery", click OK, Apply"
After Unchecking "Auto Proxy Discovery" I also had to Uncheck "Use Passive FTP Mode (PASV)" and then Restart Boot Camp Assistant.
odiggy's comment and tried restarting Boot Camp Assistant. That did the trick, even though the Auto Proxy Discovery checkbox ended up the same as it started. Windows support software is now downloading... if you're downloading to an external drive make sure there's a least 1 GB of free space because the WindowsSupport folder is around 700 MB and presumably will grow with time.
https://discussions.apple.com/thread/3223773?start=15
Also, Change Domain Name Servers to OpenDNS IP address servers - that should be at the router firewall rather than the OS but may be in both for some people. You may be using the defaults your ISP uses, save those if you want, and try these:
208.67.222.222
208.67.220.220
Make sure you are not using LittleSnitch or Symantec (probably other 3rd party firewall)