I would like to convert one of my WordPress multisite blogs back to a single install on another server. it's not the main site but one of the subdomains. Is this possible?
Use the Export function.
It's just like taking your blog off WordPress.com and switching to the .org standalone.
'Cept you also have access to the themes & plugins files.
I wouldn't bet my life on the export function. It's worth a try though, just check your data afterwards. Export doesn't move images, media files, etc. So if you have a lot of images, you might wish you used relative URLs ;-)
the codex has a great article called "Migrating Multiple Blogs into WordPress 3.0 Multisite" making it simple to follow steps that should get you Migrating in no time.