A lot of people, unfortunately. The first thing to do is make sure there are no coolant leaks and top off the system. After that, most seemed to be fixed by purging the air out of the system, but, occasionally, the heater pump fails, which pushes the coolant through the heater core. It's attached to a bracket off of the firewall on the driver's side of the engine. It'll have two wires going to it and has two heater hoses attached to it.
To purge the system, set the front and rear heaters on the maximum hot settings. Jack the front end of the vehicle up as much as safely possible; if you can't do that, find a steep upgrade and park so the nose of the vehicle is facing upward. Start the engine and run at 2500-3500 RPM for about 15 minutes, or longer, as necessary, until hot air is coming out of the vents. This should purge most of the air out of the system. Shut the engine off and lower vehicle. Check the level in the reservoir bottle, but do NOT open while the engine is hot. When the engine has cooled down, remove the reservoir cap and top off, if necessary.