First of all, you can't do this safely if you have a gas
dryer, because the carbon monoxide fumes from the gas
combustion are vented along with the hot air.

Second, I doubt the extra humidity in the room would be an
issue, because humidity is usually much lower in the winter
when you would want to reclaim heat from the dryer.

Third, there are products on the market which do what you
want. See, e.g.:

http://shop.store.yahoo.com/energysu...tdryerhea.html
http://www.cetsolar.com/extraheat.htm
http://www.usahardware.com/inet/shop...lecto/ex12.htm
http://www.audio-etcetera.com/prod.itml/icOid/107528