While I have not experienced this bug myself, two of my friends experience this as well. They can log in and host games, but their games do not appear in the lobby; nor can they join any games. They don't even come as far as the NAT screen, they are simply told that they can't join.
They are running Windows XP 32-bit and 64-bit respectively.
Being unable to play with other players does limit their ability to playtest quite a bit. I hope there is a fix on the way for this.
