Have you googled on envelope printer or envelope printing. Wasn't going to ask that as I might have to come back and fling a "Duuuuh!" if the correct answer wasn't forthcoming... ...But here it is and now you're stuck with it.
I don't know of any specifically for that purpose though I suppose someone has rigged a laser printer for it with a custom feeder for the bulk mail business - talking big bucks here. You have to watch what kind of adhesive is on the envelopes as laser heat can make some melt and gum up the works. My Canon iP3000 can stack 10 or so in its top feeder. Good luck.
Xerox, Canon, Kyocera/Minolta and possibly HP would be some places to look for the customized lasers - but you'd probably have to call a rep. Doubt theyd be noted on a web site.
.bh.