So... we would have exponential (et) growth if the #_of_days was constant... but now it's halving every time the value doubles. I'm not going to figure out the formula for that, but it seems that if this development continues, we reach the singularity in 8 days. Ladies and gentlemen, I hereby forecast that by 17th of April, Bitcoin is infinitely valuable.
Damn! It appears that reality got in the way of my bulletproof mathematical analysis. But I'm putting up a sell order for 1 BTC at $1000000 at Mt. Gox., just in case.
(Bitstamp only allowed for a maximum price of $99,999.99. Not very forward-thinking of them, I'd say.)