I just took our annual hazardous waste management course/refresher like a month ago.
The instructor said that the EPA basically doesn't regulate individuals. As an INDIVIDUAL, you're allowed to dispose of most things in the dumpster. His example was oils (i.e. from your car), though I would assume paints, paint thinners, etc. would also apply.
The more hazardous waste you produce, the more likely you would fall under EPA's purview, but most individuals don't come anwhere near that limit... A gallon of used thinner is OK to go into the trash.
But as Foster notes, your city, county, or state may have more stringent guidelines. His first suggestion is a good one, and I would probably do that if I were disposing of relatively large quantities. If it's just a bit (i.e. one of those 1/2 oz. Testors paint bottles or less), I'd put it in my dumpster. Soaked rags/contaminated paper towels... dumpster too.
Whatever you end up doing though, make sure you feel right about it.