From the TOU: http://www.craigslist.org/about/terms.of.use

u) use automated means, including spiders, robots, crawlers, data mining tools, or the like to download data from the Service - unless expressly permitted by craigslist;

If you are serious about this site and you think that it could become a largish site you should definitely get permission.

Agreed. You really need to read the CL TOS and make sure you're working with CL to use their data.

Could you name an example of this? I scraped CL for a while and they didn't say a word, and I've never heard of them shutting someone down.

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.

how popular is your site? I remember they shut down some real estate site that mashed up craiglist data with google maps that got shut down pretty quickly. Not sure what the name was.