Are there people who map cheat the fog of war away in Starcraft 2 on Battle.net? Do I have to worry about this?
Fog of war cheats ("maphacks") definitely exist.
How to detect when you've been maphacked: If you suspect that you have been maphacked, load the replay. Watch the game from the perspective of your opponent. If he appears to be spending lots (abnormal amounts) of time hovering his screen over your base in the fog of war, chances are he is hacking, as in the following video.
This technique has been used to expose hackers quite a lot in the BETA phases. If you are able to detect the presence of a maphacker using this technique you can use the "report user" feature within battle.net and post an explanation in your report.
Yes, map hacks have been available since early beta.
However, they aren't common, and most if not all publicly released map hacks are easily detected. There likely are unreleased map hacks that are still undetected, but it's unlikely that you'll encounter them on Battle.net.
BLizzard continously monitors the user's process list, so if they find anything unusual, like a memory debug/editing tool, hacked game content etc. they simply ban the user.
The problem of map hacking has been overhyped since before Starcraft 1. I would not be seriously concerned about it.
Certainly people will accuse each other of map hacking, but these accusations have never been verified. Like alligators in the sewers, I'll believe it when I see it.
According to this: http://www.teamliquid.net/forum/viewmessage.php?topic_id=140168
They are map hacks. This is extremely sad because when they announced SC2 required battle.net to play I assume this was to prevent map hacking by hosting the game on the server. But alas no. Blizzard has failed.