This is mostly going to be guess work, have salt at the ready.
The economics of Bitcoin are obviously a matter of supply and demand. Let's try and break it down a little :
Mining obviously does something to the tune of 365 * 24 * 6 * 50 = 2`628`000 BTC a year. This is valid for 2012, next year they're going to be looking at half that, or about 1.3 mn.
SilkRoadi. Nobody here is actually a drug user so it's quite impossible to speak with much confidence, but it'd seem SR is mostly a pot&pills thing. Whether you can parlay something like that into 2 mn BTC yearly volume is dubious. 1mn a year seems a more likely figure, but even this may be significantly inflated by the fame effectii.
Gambling. SatoshiDICE cleared something on the level of 15k so far this year. On yearly-adjusted figures (since "this year" only means about 6 months) and accounting for variance that would be about 25k yearly. The total BTC employed for gambling on SatoshiDICE would then reasonably be something like 50 to 100k which would probably be cycled quarterly let's say. I'd guess all the rest of gambling together just about doubles SatoshiDICE's impact, so figuring everything in together would be something in the 500k BTC range.
Finance. Monthly volumes are consistently in the 50-100k range on MPEx. The rest of the finance sites seem to account for maybe another 30-50% on top of that. This could be said to mean about 1.2 mn BTC yearly, but then it's improbable that all that volume actually settles in the blockchain. Judging the impact of finance in the 500k BTC range would seem to me reasonable.
Retail. Judging on the recent Black Friday alone I would venture a guess that the overall BTC retail volume is somewhere in the 100 to 200k BTC range. (As an aside : some think this field has the best perspectives for growth, I can see their arguments but am unconvinced.)
Store of value. The supposed "useless" use of BTC, I for one would guess this accounts for anywhere between 5 and 10 mn yearly volume. Let's say, as a round figure, 8.
This all comes to about 13mn BTC a year. Time for a pie chart :
On the other hand, there's the fiat-to-BTC exchanges. For the past 365 days MtGox has traded about 6 mn BTC, with the rest of the exchanges together trading another 2 mn BTC. This leaves something like 5 mn BTC to have been traded OTC, person-to-person, cash-in-mail and through other such arrangements. Time for another pie chart :
Sad bee of BTC says hi.
And finally : the total BTC in circulation went from 8.5 mn BTC a year ago to 10.5 mn BTC now, which averages to 9.5mn, and implies a yearly movement speed of BTC slightly above the unit : 1.37.
All this brought to you by the MPEx hunches department. Enjoy.———