WordPress MU

Is it as easy as just going to Site Admin-->User Blog-->Edit, and changing all instances of the previous subdomain name to the new subdomain name?

My blog owner has been using her blog as a "sandbox" and keeping it private, but she wants to "go live" and change the subdomain address.

I've been searching around and the only reference I found to this was a thread from a year ago and it talked about having to go into MySQL and changing some entries in there manually.

Yep, that should do it.
Site-Admin -> Blogs -> Edit (next to the blog)
Make sure you get the Fileupload URL one towards the bottom - I've missed that a few times.

Excellent, thanks for the confirmation!

Any links within posts though will need to be edited. That's where the manual mysql edits come into place if you want to follow that route.