It doesn't appear serious, and Mike McCarthy said that QB Aaron Rodgers wanted to practice anyway on Monday. From Packers.com:
Quarterback Aaron Rodgers sat out Monday’s practice as a precautionary measure as he deals with a foot strain. McCarthy said Rodgers’ foot "flared up on him this morning," but he did not rule Rodgers out of playing on Thursday.
It's the first I've heard of it, but I'm not surprised he's got a minor injury that isn't severe enough to be reported. It sounds like something he's been dealing with for a little while, and if that's the case, it hasn't hindered him during the first three preseason games.
Unfortunately it's hard to imagine the injury getting better any time soon. Unless it's fully healed this week, he'll be running on the injured foot every day for the next few months. On the other hand, this is football, and he's in no more danger of aggravating a foot injury than suffering any other injury.