It has been dry for a while, but when you consider that out of six games he didn't score in since Blackpool, only two of them were the full 90 mins, one was 64 mins, and in the other three matches he was subbed on at 66 mins, 76 mins (and assisted a goal for Rooney) and 69 mins respectively. So while it may appear to be over a month and a half of a drought, or 6 matches, he's actually played less than three and half matches worth of minutes - you can't score if you're not on the pitch. Three and a half matches is not that big a deal.
What you're saying is certainly valid but I'd also like to point out the same fans use these facts to point put the ineptitude of Tevez and Owen but in reverse, using these appearances, albeit short as appearances in which they don't score, yeah?
What I'd also like to say about your point is the reason Berb wasn't on the pitch was because he wasn't scoring in the games prior.