Valve is keeping those people and LAN shops with older computers in mind, but they haven’t announced any official minimum specifications yet.
Many fans of Heroes of Newerth stated that they prefer it to DotA because of the graphics. Unfortunately, some people are unable to even try HoN because their computers can’t handle the game.
So, I’m glad to hear that DotA 2 should be accessible to those with lower PC specs. I wouldn’t want anyone to be left out.
(Source: Official DotA 2 Blog)