This was interesting from pg's Viaweb FAQ:
http://www.paulgraham.com/vwfaq.html
"What database did you use?
We didn't use one. We just stored everything in files. The Unix file system is pretty good at not losing your data, especially if you put the files on a Netapp.
It is a common mistake to think of Web-based apps as interfaces to databases. Desktop apps aren't just interfaces to databases; why should Web-based apps be any different? The hard part is not where you store the data, but what the software does.
While we were doing Viaweb, we took a good deal of heat from pseudo-technical people like VCs and industry analysts for not using a database-- and for using cheap Intel boxes running FreeBSD as servers. But when we were getting bought by Yahoo, we found that they also just stored everything in files-- and all their servers were also cheap Intel boxes running FreeBSD.
(During the Bubble, Oracle used to run ads saying that Yahoo ran on Oracle software. I found this hard to believe, so I asked around. It turned out the Yahoo accounting department used Oracle.)"
http://fallenpegasus.livejournal.com/tag/s3
It's still at a very early stage in terms of implemented features, but he seems to be moving forward rapidly.
I have no idea what kind of performance he's getting -- I'm planning to check out the MySQL source sometime next week and do a test build. Extremely cool idea. :-)