Can we interpret a rolling average transaction count per block as the 'adoption curve' for Bitcoin?
The main problem I can see with using such a metric to measure adoption is that a tremendous amount of transactions take place on the various exchanges during periods of heavy speculative investment.

During the current downtrend we're probably experiencing a lower percentage of traffic from investors and a higher percentage of traffic from actual users of the currency. The problem is that there isn't really any feasible method of weeding out the MtGox, TradeHill, Britcoin etc traffic from the regular use cases.

The only way that I can imagine being able to weed out such transactions is if MtGox and their ilk actually provided a full list of the transactions they process each day. Armed with a list of transactions from at least the major exchanges we could mostly eliminate speculative use from our measurements and see something closer to actual adoption levels.

Unfortunately to my knowledge no such transaction list exists and without it, it's uncertain if such a rolling average is showing a decline in actual adoption or simply a decline in speculation. One of the big difficulties to be encountered in Bitcoin is that it's simultaneously quite open and extremely closed and can be very difficult to perform meaningful analysis on.