As much as I enjoy the Raspberry Pi, I doubt if it could handle the traffic or if it would last long with the limited write cycles of an SDcard.
For example, what pages get accessed the most anyway? I'm guessing the latest source code and maybe the latest release, though most people probably just apt-get git instead so it's probably mostly the source code. Then there are man pages and some other info pages, if I remember correctly. Sounds like the latest release + 90% of those text pages can easily fit in RAM. So memcached? Nah, the Linux kernel happily caches the files that you read from disk.
I don't know the actual numbers but it doesn't sound infeasible to me. A $230 hosting bill is very heavy though, I guess you'd need some serious fiber as well to provide the uplink. But again, without numbers it's all "maybe" and "probably".
Nowadays I would have used a VPS for that. The point with S3 or any other cloud solution is that they make sure you're up and running. Even though it might still be useful, the need for good service monitoring is as good as gone when moving to one of these cloud based platforms. And then I'm not even taking into account the time you have to invest in setting up and configuring a RPi properly vs just pushing a repo to Github Pages, or uploading a zip file to S3.
Heroku or other cloud platforms can be crazy expensive, but for static file hosting, S3 or Github Pages is more than enough and quite affordable.
That said I'd agree, Raspberry Pi's a great but not quite fast enough for serving a high-traffic website.
I've just checked and it looks like they nolonger do so though...
$ curl -I https://www.raspberrypi.org 2>/dev/null | grep X-Served-By
X-Served-By: Blog VM 2