When to shift into "short stack" mode

I have heard many different opinions on when your game should shift into "all in or fold" mode.

Harrington deals with this in part two of his book... with his discussion of "M" and the various zones you might be in.

I've also heard other pros talk about shifting to this strategy when your stack is less than 10xBB... yet some say this is too early.

I'd like to hear other opinions. How long do you wait until you are are only looking for hands to push all in with?

and a side note... are there exceptions to this rule? when you have only slightly more than your requirements for an all-in or fold strategy... do you go ahead and push in with middle to small pairs? These hands are increasingly unplayable as the blinds increase, and there are fewer limpers... and they're too marginal to raise with (too many overcards could come)

I've found myself with about 11 times the big blind... just pushing all in with a hand like pocket 8s or 9s... (as long as I'm not under the gun at a full table... too many people to get through...)

I consider myself a pretty good post flop player, but the combination of the strength of my hand, (too strong not to play - that pair is likely to be the best hand at the table) my stack size, (too small to make moves with, and needing to amass chips to contend for final table) and the low probability that I'll be able to limp with 4 or 5 others at a later stage of the tournament... all tell me that pushing all in is probably the best play...

other thoughts?

I usually shift into an all in or fold mode when I get 10 bb's or less, but will not go all in with the range that a stack with less than 5 bb's would. I would rather double up and get to or above 20 BBs where I am most likely very comfortable than to get down to 5 and get up to 10 where I still don't have a stack where I can make postflop plays.