PSR distribution (taken from HoN forums)
PSR:
% > 1800: 3%
% > 1700: 16%
% > 1600: 45%
% > 1500: 68%
% > 1400: 83%

Mean: 1547
Median: 1579
Mode: 1662
Standard Deviation: 171
This is a distribution of PSR, not MMR, though the two seem very correlated in my personal experience. The data came from:
http://www.heroesofnewerth.com/players/ which for some reason doesn't include MMR, so I can't compare the two.
The median isn't 1500 due to stat resets, mostly done by lower players who essentially gain free PSR.