Nothing?
This port is only used internally (in the case of Plugwise and other local webservers). Right?
I use port 80 for website, weblog, IP cams, etc. etc. but all have a different local IP.
You're addressing Plugwise with a combination of IP and port, not just the port.
As long as that combination is unique, you don't have a problem.
Just like there are more streets with the same housenumber...
