In his post-practice press conference today, Mike McCarthy revealed that both Nick Barnett and Donald Driver had knee scopes earlier this offseason. I have not read a transcript yet, but it sounds like Barnett had one knee done while Driver had both knees scoped.
This explains why both players have been standing on the sidelines during most drills so far during OTAs. It is interesting to note that both players were running without any noticeable problems at Driver's charity softball game. That is incorrect. Barnett didn't run (see update below) and Driver's kid was running the bases for him. My bad.
UPDATE: Thanks to 'Erik' in the comments section for pointing out the following from over at Packers Lounge:
They did not let Nick Barnett run at all. I found that odd. Is there something we don’t know?