Craigslist has apparently recently added an email alerts option. Unfortunately there is no way to set them up for multiple searches at once. However, our Direct Results view already does combine multiple cities into a single search, and the entire US into only 20-something searches, so if you were interested, you could run a search on SearchTempest using our Direct Results view, then inside each craigslist results page, use the email alerts option. Would be a bit of effort to set up, but might be worthwhile depending on what you're searching for.
Alternatively, we can help you set up RSS notifications for your search, direct from craigslist and eBay, for all the cities you're interested in. See the RSS section for more details on that. We've written detailed instructions, so it should be easy to set up, and should give all the benefits of email (and more, since RSS is made for this kind of thing!)