The innternational residential code requires dryer vents to terminate outside the building, with a couple exceptions, such as for condensing dryers where the manufacturer has designed it to work without a vent.
http://www.greenbuildingadvisor.com...ing-code-questions/18918/dryer-venting-inside
The two big risks I can think of are humidity causing condensation (not likely to be an issue if the humidity is normally low, and it's not discharged into a relatively small, closed room), and either airborne lint to breathe in or settle everywhere or lint build up in an extra trap like those off-the-shelf indoor vents have. That's is a fire risk, but mitigated if the user cleans up any lint - something you already have to do with the trap in the dryer.
Taking into account those concerns, it doesn't seem like a big deal to vent indoors as long as you restore it to outdoor venting if you ever sell the house so an inspector doesn't flag it.
I'd never do it in my area because of the humidity.