I believe what you are looking for is the [Broodwar API (BWAPI)](http://code.google.com/p/bwapi/). Although not an official SDK, it pretty much allows you to do anything in Starcraft: Brood War.

It even has a [yearly competition](http://eis.ucsc.edu/StarCraftAICompetition) supported by Blizzard.

PS If you decide to use it, don't forget to check out two very important extensions:

 - [The BWAPI Standard Add-on Library (BWSAL)](http://code.google.com/p/bwsal/)
 - [The Broodwar Terrain Analyzer (BWTA)](http://code.google.com/p/bwta/)