I think that Fletcher the A.I. General from Defense Grid would make an excellent announcer for Dota 2. It's a authoritative British voice that is also a bit charismatic.
The character already serves as an announcer in his own game, talking about the enemy's status and about towers. He also comes from a game studio who already works closely with Valve.
The character is an AI and his voice does have a filter on it to sound computer generated, but it can work for him if it can work for GLaDOS as well.
It's not the best video to showcase, but it's the best sample I could find on YouTube: http://www.youtube.com/watch?v=Rf7ap_x7ZEc