There's a lot of servers out there that you can easily find through the Minecraft Wiki or other forums.
Resetting is done when new terrain features are introduced. Old maps wouldn't get them if we didn't reset. Since the game's been officially released now though, we probably won't reset too much.
You can't on the server. We backup maps each time we reset though.
Probably impossible, but I think multiplayer server data can be used as singleplayer data, so if you ever build something on the server and would like a copy when we reset, I guess I could provide that to you.