Is there a way to check past difficulty levels? Or did anyone take note of what it has been? I'm going to attempt to keep a log over the next few days to see if we can get an idea of where the difficulty is going (and therefore what the reward/inflation rate will be). Also this should be useful in determining the compute power of the network.

Currently it sits at d=7.23843342. This jives well with the block reward I just received (r=19.07).

(999 / d^2) = r <=> d = sqrt(999/r)

So, with a reward of 19.07 we have d=7.23781.

In the time it took me to write this, the difficulty changed to d=7.24123460 in about 15 minutes. So, if my math was right the difficulty increased by almost 0.04% in 15 min or almost 0.003% per block. Using both of the blocks I found gives around 0.002% difficulty increase per block. This is all assuming a one minute block time, so take this all with a grain of salt.

According to Bitcoin Magazine, a doubling of computer power would cause a 0.02% increase in the difficulty of the next block. I'm off to bed!

source: http://bitcoinmagazine.com/primecoin-the-cryptocurrency-whose-mining-is-actually-useful/