Next I want to endorse Christof's note of caution. We need to treat 40 Hz as an hypothesis, and only as an hypothesis. Historically in neuroscience, this kind of situation has come out both ways.
New methods can sometimes be treacherous, but it is equally likely to be especially revealing.
We simply do not know. We probably won't know until efforts to replicate the MEG data are in. Maybe Kevin can fill us in on that. As an educational exercise I think this discussion is useful no matter how the hypothesis turns out over the next few years. One reason is that oscillatory phenomena are abundant, as Kevin and Jim have mentioned, and if the 40 Hz hypothesis fails it is still a useful exercise to think through carefully how oscillations could be involved in analysis of input, storage and retrieval, in eliciting neural output, planning motor output and control, and coordinating populations of neurons.