If you visit http://crgslst.com/ -- a multi-city search site -- it pops up a message about how they were blocked.

They also disabled Jeff Atwood's multi-city search: http://www.codinghorror.com/craigslist/ (he posted the letter he got from them)

Then again, http://www.housingmaps.com/ is still functional, and might be the most popular Craigslist mashup -- I'm assuming they have a deal in place with CL.