Focusing - well in servo mode, having all 3 points active is not a good idea with the D60 anyway, but that doesn't really explain the problem in single point mode.
I have experienced something similar on a few occasions over the last year (maybe 5 or 6 times). For me, the solution has been turn camera off, remount lens, turn camera on. Everything normal.
Complete camera lockup I've had once - which in my case I think was partly user error - snatching out a CF card before write was complete. Like you, taking battery in/out solved problem.
How serious is this? Not sure - I've also had the focus problem with an EOS 3, so I don't think its D60 specific. I will agree the glitches seem to occur with more frequency with my DSLR than my SLRs, but then my DSLR use is much more intensive.
Also, if we think of these cameras as computers - which they are to a large degree - then the occasional crash is perhaps not too surprising. I see taking out the battery and putting it back in as the equivalent of [ctr][alt][del].
If I had to reboot my D60 everytime I did a shoot I'd be worried. As this seems to be a fairly rare occurance, and no other ill effects arise, I'd be inclined to just shrug it off ... and perhaps wish my PC
were as reliable as my D60
Colin K. Work, Pixstel