This would allow deflation to be calculated from the block chain?  The equation is

MV = PQ

M = money supply (= 50*block count)
V = velocity (assumed constant?)
P = price level
Q = quantity (assumed equal to number of transactions?)

P = (50*blocks)/(number of transactions in the last 3 months*)

I don't think that would work very well.  Someone who spammed transactions could effectively push the P value down causing fake inflation.

Maybe a better formula would be

P = (50*blocks)*(median transaction size)/(total transferred)

This is still potentially a problem, if everyone sent 2 transactions instead of 1 for a month, then it would give a result of 100% inflation.

Another option would be to only include transactions that pay at least 1% of their value in fees, but again not really that solid.

Anyway, ideally, there would be a deflation multiplier that any client could calculate.  This wouldn't affect the bitcoins themselves, it would just be a well defined way to convert bitcoin values into a stable value. 

You could agree to pay someone 1 "stable-bitcoin" per month and the effect would be that you pay 1, 0.98, 0.96, 0.941 .... every month (assuming the deflator was 2% per month). 

The idea would be to make it easier for people to think in bitcoins.  Currently, people would have to keep recomputing "fair" prices on a month by month basis.  At least with current fiat currencies, you only need to do that on a year by year basis.

People who have bitcoins in their wallet would see their stable-bitcoin totals increase by 2% per month (or whatever).

Finally, I don't think it is a good idea to compress all deflation discussions into one thread.  Pretending that a problem doesn't exist doesn't make it go away.

*:  The time period could be defined using number of difficulty updates or something.