I would also love to see UPnP support for mIRC
I think it would be fantastic for those of us who don't have admin access to their routers, or those who get their internet access from dormatories and can't manually configure ports. As for concerns about security issues that have been raised in the past, here is a suggestion: leave it off by default. People who accept the security risk can enable it if they choose. Personally I think it is a fairly benign but vastly useful feature.
edit: Something occurred to me after I posted. Actually I think UPnP could potentially increase security in mIRC by allowing the use of randomized ports each time it is used. Also you would not need to leave port 113 open all the time (as is required now). UPnP would simply open it when needed, and close it when mIRC is terminated. I'm sure even Steve Gibson would like that (Port Probe