Lindsay Lohan Cries on Letterman

Lindsay Lohan mixed vodka with rehab -- with a splash of tears -- on David Letterman's show last night.

The most famous defendant since O.J. Simpson says she's taking her impending rehab seriously ... but then -- on camera -- she reached for a coffee cup filled with a mysterious liquid.

Best mug shot ever!

By the way, Linds kept the party going after Letterman -- she hit up 1 Oak Nightclub, and was seen leaving early this morning.