There was a case a few days back of a user going crazy and starting edit wars in all of his 42 questions, resulting in 50-100 new revisions per question. It being the weekend, it took 1-2 hours until a moderator showed up to suspend the user.
While no real damage was done, and in hindsight instead of starting an edit war, we could have just waited for a mod to appear, I can think of cases where a user going crazy could be a real problem even for such a short time. For example, somebody posting really, really bad graphical content - say, snuff, hardcore porn, or worse. There is also the possibility of a Firesheep hijacking of a high-rep account.
Now, it taking some time until a mod can look into a flag is perfectly normal. Moderators' time zone coverage will increase over time, but you can't expect emergency-service reaction times around the clock.
I would therefore suggest the ability for three 5k+ or 10k+ users to vote to "emergency suspend" a user for, say, three hours, until a moderator can look into it. The function could be located in the 10k+ tools, visible only for user profiles that have had a recently flagged post that wasn't acted upon within 10-15 minutes. Flags would wither away quickly if three votes are not accumulated within a defined (short) time span. An emergency suspension can be cast only once for a user, after that a mod has to look into it.
Obviously, it would have to be communicated clearly that this is an emergency brake only, and that misuse of the function would have grave consequences. Making it a three-vote thing takes away much of the potential for misuse, though.
Edit: Shog9 makes a number of very good points pointing out that this is solvable already through coordinated flagging, and doesn't really need a new, potentially misusable tool. I tend to agree. Should this be deleted?