There are a few urban legends out there about licking envelopes. The first one I heard was the one about someone licking an envelope that was laced with cyanide on the glue and the person died from it. The second legend is the one about the woman who licked the envelope and her doctor ends up pulling out a live cockroach from her tongue. Of course, these have all been verified as urban legends and are not true.
Are there real health risks to licking envelopes? I think the only real risk may be if your Mailroom has a preference on whether you seal an envelope or leave it open to go through the postage machine to be sealed? They may be annoyed if you don't follow their procedures, but other than that I could not find any health risks associated with licking the glue on the envelope.
I personally do not like licking envelopes because of the taste of the glue and because of the possibility of a papercut. Those papercuts hurt!
Here is a new one I heard: "There is a Calorie in every lick of the envelope". I will leave that to you to investigate.
P.S. Before passing on these urban legends, check on http://www.hoaxbusters.com/ to verify if it is true. E-mail people at their home e-mail address, not work and ask before sending. Not everyone appreciates getting these types of e-mails.