I also posted this on the wordpress forum, so there's no need to respond in both places. I'll post the solution in both places.
I have just setup a new multisite install with subdomains at socialsnakes.org
On the network admin dashboard, everything appears to be working correctly, so I added a test site at melissa.socialsnakes.org. When I try to access that site's dashboard (http://melissa.socialsnakes.org/wp-admin/), I get the following error:
The requested URL /wp-admin/ was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Hunting through the forum here and at wordpress, it appears that dreamhost does not support wildcard for shared hosting, but I should be able to set up subdomains manually. I tried to do that with the subdomain mirroring the main site. Suggestions?
I haven't uploaded any posts or pages to the new install and it's still set on the default Twenty Twelve theme.
Other information that might be informative:
WordPress version: 3.5.1
Host: DreamHost (shared hosting)
Domain registrar: Hover
Browser: Firefox on Ubuntu 12.04