Check here: Full Titles and Runner-Ups Thread --- Big Collection of Stats!
All main tour finals 1968-2010 Wimbledon added. At the bottom you can see 65,3 % of best of 3 goes in straight set and (only) 47,3 % in best of 5. But this is finals only.
Now counting all ATP matches* since 2000:
In 23651 completed best-of-3 matches, 15530 matches went 2-0. (65,7 %)
In 5205 completed best-of-5, 2578 matches went 3-0. (49,5 %)
*Excluding Davis Cup and World Team Cup