It’s rather powerful: as long as the origin airport is one of those shown on the map, and the destination airport is reachable via a domestic flight on one of the major airlines (AA, AS, B6, CO, DL, F9, UA, US), the search results come back instantaneously. Multiple origins and destinations (up to five of each) are supported as well: http://www.google.com/flights/#search;f=LAX,SFO,SEA,LAS,DEN;... And the results stil…
It doesn't take a tremendous amount of computing power -- remember that most airlines are still running ancient mainframe reservation systems.
The multiple destinations functionality was from ITA -- it's been around for years. Check out flyertalk.com for discussions -- you can input multiple layovers to have it compute cheap fares that maximize miles/segments. They keep a basic backend open at: matrix1.itasoftware.com