I second raybay.
It might be safe to vacuum once, maybe twice.
But there is still that chance for your vacuum job to fry the components in a PC, even if its 1/1000 chance.
There was a post on Techspot before about how to vacuum your PC safely. It had a full explanation on how static builds up, how it discharges, how it destroys components, and of course, how to overcome static damage.
From what I remember, the easiest way is to make sure either both vacuum and PC are grounded, or both aren't. Most vacuum cleaners are grounded, and most PCs are grounded too. Most damage occurs probably from people putting their PCs on carpets (ungrounded) and then vacuuming it (grounded).
So, just leave the power cord plugged in, and vacuum it, so both are grounded. Just make sure that your PC is switched off.