I found some statistics on the USPTO website regarding ex parte patent re-exams.The date range is from 7/1/81 – 9/30/10.

a. Number of re-exam requests granted:92%

b. Overall re-examination pendency (filing date to certificate issue date):25.5 months

c. Re-exam claim analysis (requester is a 3rd party):

***** All patent claims confirmed = 25%

***** All patent claims cancelled = 13%

***** Claim changes = 62%

http://agoracom.com/ir/edigital/forums/discussion/topics/461402-774-patent-re-exam-ordered/messages/1494666#message