Yes. At a technical level, CloudFlare is a caching reverse proxy provisioned via DNS.

Does it serve cache hits via a CDN, or do you provide all the bandwidth?

I hadn't thought of this before reading this thread, but a great service would be something like Varnish that's backed by some CDN instead of a big block of memory. That way, we can get great performance out of sites that are low on memory and bandwidth, like your low-end Linode or Slicehost.

That's a good description: Varnish + CDN (+ security). CloudFlare ends up making sites twice as fast and saving sites about 65% of their bandwidth. We run our own CDN with nodes scattered around the world.

Sounds excellent. I may be trying out your service in the near future!

While that does sound awesome, wouldn't this be pretty easy to accomplish with some of Amazon's products?

In addition, based on their size, they effectively are a CDN!

It's pretty easy with open source products ;)

Only if you have your varnishs/mod_security in several hosting locations around the world.