Ok, just going to toss in my 2 cents here..
I am not really familiar with XMPP, Xport or WiPort, but as for letting Arduinos talk to eachother over the internet..
The way I would do it would be to make a simple (remember, always follow the KISS-rule;)) Visual Basic program that works as a proxy between IRC and the Arduino.
Talking to the Arduino using VB and the MSComm-object is a 3-minute project for anyone who has done it before, and connecting to an IRC-server isn't exactely rocket science. The VB-program would really only need a simple text-file for configurations, containing:
- nick/alternative nick
- Channel (for messaging other "groups" of Arduinos)
- COM-port number (for the Arduino)
- baud rate
Of course, the VB-proxy program would need to "translate" a bit between IRC-protocol and whatever format you want to send data to the Arduino with, but building a simple protocol on top of IRC isn't hard either.
Arduino_1 sends "msg Arduino_2:set 1 on" through the serial link to the computer, and the computer sends to IRC "/msg Arduino_2 set 1 on".
The VB-proxy logged on to IRC with the nick Arduino_2 recieves something like "PRIVMSG set 1 on" and will forward this to it's Arduino.
For addressing groups of Arduinos, one could simply msg a channel and every arduino in that channel would recieve the same message/request/data/whatever.
Like I said.. perhaps a bit of topic, but my 2 cents none the less. :P