This was added as a player note in Yahoo yesterday:
May 20 The Denver Post reports Colorado Rockies OF Preston Wilson (knee) is slowly building up strength in his left knee, leaving his return near the last week of June more likely.
Sounds to me like the date is consistently being pushed back (this is the second time, it was originally late May). Considering the fact that the surgery wasn't as bad as they thought it would be, that leads me to believe he's just a slow healer.
He might as well just aim for after the All Star Break, so we're not disappointed.