How do I move existing pages into a folder?

Sure, just go to the page, then look on the right-hand side, under the 'Search' box. There's a link that says 'Put this page in a folder'. Click that link, and pick a folder.

There's no way to put pages into a folder en masse.

Ah, there it is... I was looking on the "All Pages" screen and the "folders" list on the sidebar... however, that leads me to my next question - short of opening every page or folder, there doesn't appear to be a way to determine which pages are in which folders... I'm thinking some sort of site map display would be very useful, especially as the wiki grows.

"I'm thinking some sort of site map display would be very useful, especially as the wiki grows."

Yeah, we used to try to do that client-side, with Javascript, in V1.0, but with folders, V2.0 has gotten a lot slower, so they really need to start doing some of this stuff server-side. I wouldn't count on it anytime soon, though.