Thanks for the answer!
Another substitute for your tongue... at Office Depot, you can get a small plastic bottle with a double-sided, slatned sponge as a cap. You just fill it up with water, put the sponge-cap back on and start slicking envelopes! Saves your tongue from paper cuts too and only costs $0.79 a bottle.
Edited:
ps - just saw AtlanticVA's comment... that's an even better idea.