I recommend going with Cairo. It has an interface very similar to <canvas>, and is really convenient for 2d graphics. I've used SDL before and am not a fan. If you want to support multiple OSes, you can save some time by delegating cross-platform window and input handling to GTK and just using the Cairo canvas it gives you (that's how the McCLIM GTK backend works).

http://cliki.net/cl-cairo2 is the library to use, the original cl-cairo seems to have disappeared

http://www.cliki.net/cl-gtk2 has GTK and Cairo bindings

There's other GTK bindings listed at http://www.cliki.net/GTK%20binding

Also, it appears someone has tried implementing a Morphic UI in CL before: http://www.cliki.net/MorphiCL

Vladimir

PS - Sorry for breaking threading with this reply; I've switched off digest mode and this shouldn't happen in the future.