So, it's possible to publish a blog or other static website, built using ikiwiki, without needing your own web server at all. Ikiwiki builds a website and uploads it to Amazon, which then handles the web serving for you.
If you want a traditional wiki that people can edit online, you can still serve the pages out of S3, but you will need to find a "real" web server to host the ikiwiki CGI that handles the page editing. It'll then inject modified files into S3 as necessary.
Amazon EC2 would be the obvious choice for where to run the "real" web server, but probably not the easiest one to set up. In my experiments, I've been running the ikiwiki CGI on nearlyfreespeech, and serving the rest of the wiki out of S3. Since page edits are relatively rare, I estimate this approach will cost a dollar or so a year for the CGI hosting (most of it paying for disk storage). The Amazon S3 hosting of course depends on number of hits and storage size. And presumably it will scale very well, and be very competatively priced, if you believe Amazon's marketing. :-)
I'm loving that the design decisions I made about ikiwiki at the very beginning -- that it would use static web pages, and would be backed by a real revision control system, is now letting it be deployed in these interesting ways that I did not begin to envision back then!