Just tried this when I was tired of licking Christmas card envelopes. I got out my daughter's Aquadoodle marker. It was incredibly quick and efficient:) You can buy them at most box stores without buying the Aquadoodle set with mat. So even if you don't have one it would be worth the investment.
If you've never seen an Aquadoodle, it's a great toy — a white mat that changes color when "painted" with water. The marker that comes with the set is basically a felt-tipped pen you fill with water. Ours is floating around the house somewhere.
Other ideas for dealing with envelope-sealing fatigue: get your kids to do it, or forgo envelopes altogether. My holiday "cards" are usually letters I fold into thirds, staple, and address/stamp/send as-is.
More: Christmas hacks