Unless it has dried to a solid, you need to take special precautions; tossed in the trash, paint solvents can eventually contaminate groundwater. You can speed up the drying process by putting cat litter in the can. Or go to earth911.org
for links to programs in your area that recycle paint by giving it to schools or charities. The site also lists the 2,000 household hazardous waste centers in the country that accept all sorts of hard-to-dispose-of stuff, from batteries to cleansers.