I know this is an old question but even so, 8 months later and it's still a very good one (in my opinion).
A fair bit has changed and now the game is out of beta and the ranked ladder system is in place. Each player has a solo and team MMR rating which is displayed on their profile.
It's now a lot more transparent in terms of how we are placed in matches.
This is the rough distribution according to valve prior to launching the ranked match making system. http://blog.dota2.com/2013/12/matchmaking/
5% 1100
10% 1500
25% 2000
50% 2250
75% 2731
90% 3200
95% 3900
99% 4100
Keep in mind that these values are subject to a lot of change as more ranked matchs are played. E.g. the top 1 percentile are probably more towards 5k+ MMR at the current date.
This article on reddit is a very interesting read. http://www.reddit.com/r/DotA2/comments/1y44o4/matchmaking_and_ratings/
Basically someone has created some bots that when added as a friend record their ranked matches and return the data to him. I believe the data is slightly skewed to better players as their average rank is around 3700.