What are the rules for copying "classic" maps from other risk sites?  Is it enough to give the original creator credit, or should one only re-create a map with permission from the original creator?   If so, and the creator does not create the map here and declines to give anyone permission to do so, does that mean that we may not play that map here at all?  What about making changes to such a map?

BlackDog

It's basically a hazy area but I would suggest that you do not take a game directly from another Online Risk Site. That is wrong, at least to me.
I take my map images from search engines and the like but I make sure that they are not already from Risk sites. I actually found the perfect image for a map I wanted to make but it was from somebody who was making it into a map for a Risk site and wanted to brag about it on Flikr or whatever.
This is the opinion of just one monkey but I know a lot of other players who will agree with me. I took Duck Hunt directly from the NES game but made it into something totally different. Taking a map and recreating it here is icky, IMO.

Make your board as original as possible.

Themes get borrowed and adapted all the time, but just make sure that the "art" of it isn't just a straight rip-off of someone else's original work.

Pretty much what Risky and Toaster said.
My Super Mario World map I completely redrew from Google images, but followed the basic layout of the game.
Doom has a great Mario map on WF, but I also paid close attention to not steal his map (his layout isn't from Super Mario World from what I can tell and have seen). Even though his map could probably be thought of as an "inspiration" I used the Mario World layout/areas as a guide for territories/bonuses.

As for taking board designs from other Risk sites, IMO you should definitely redraw them to your own style so that the "art" is really yours, and try to expand on it wherever possible with either the graphics and/or the gameplay. If the original designer is here, I'd ask them to port it over and/or offer to help them with the port.