Facebook post: http://www.facebook.com/topic.php?uid=195383023822358&topic=288
Credit goes to Marcos Lontra: http://forums.steampowered.com/forums/showpost.php?p=23471919&postcount=41
To solve the issue with Linksys WRT54G (Firmware Version: v8.00.8):
1-Access the router page (like http://192.168.1.1/)
2-You will see the tab Setup, sub-tab Basic Setup
3-Change "MTU: Manual"
4-Change "Size: 1458"
5-Click the Save Settings button
This is it, enjoy the game.
Good luck!
Also +rep Marcos on the Steam forum if you have a forum account set up!
So far 4 confirmed fix between this and the Steam forum.