It's not recommended to vent into the attic, although it is done alot. You are exhausting moist air - anytime except the summer, it will condense in the attic, wet the insulation, and just move the problem.
There is usually enough leakage in a house so you won't run into a problem running an exhaust fan. IF the house is truely very tight, it could be a problem (but I doubt it). If you wanted to do it high-tech, you'd put in a heat recovery system - this would extract heat from the outgoing air, and use it to warm the incoming air. Neat, but not usually necessary unless you have people showering all day.
At least one company makes a fan control with a humidistat. It runs the fan only until the humidity drops to the specified level - this way it doesn't run forever and waste your heat and/or a/c. BTW, the heat recovery system also moderates the incoming air in the summer - it cools it as the exhaust goes out.