I wrote a long description pertaining to NAT and outgoing DCCs for the guys over at BroadbandReports. I thought it would be applicable information here: http://jdc.parodius.com/irc/dcc_nat.html
Semi-technical users that don't own a D-Link 614+ should at least be able to get the jist of what the problem is (as I describe it in laymens terms).
Please don't ream me for describing the actual IRC protocol portions in simple terms -- being as I'm one of the authors of the hybrid/ratbox ircd SSL code, rest assured that I understand exactly how it all works, but just wanted to print a simplified ersion that people could understand better.
Have fun, and let me know if what I wrote up helps.