With previous versions, the connection code would randomly pick the ports to listen on.  The disadvantage of this was that you couldn't open and forward those ports manually in your router.  With Beta 3B, we included a .NET application that will let you set registry keys to specify which ports the connection code will use.  The file is ImpulseReactorOptions.exe and it needs to be run as an administrator on Vista. 

Once you enable the option, you can open and forward the default ports in your router, or change the ones specified in the registry.  If you and another player are on the same LAN, you will need to specify different ranges of ports.  Unfortunately, at the moment the connection code may require up to 10 ports and it goes every other port so you need to specify a range of 20.  We plan on improving this in the future.

Edit: There's a bug in Beta3B where the default ports list get cleared after the first round of NAT punchtrough completes and after that it goes back to using random ports.  All the IT people have gone home so I can't put up an update tonight but I'll try to get one up as soon as possible.