My mining calculator at

http://www.coinish.com/calc/ uses the last 10 difficulty values to compute an estimated difficulty change per day.

Since it assumes that difficulty grows exponentially (which is currently not a bad estimate, even though it will taper off) you can calculate the weekly or monthly difficulty increase by simple exponentiation.

(of course, actual difficulty adjustments are made every 2016 blocks, so there is no daily increase, this is just an approximation)

Current estimate for daily increase is 1.9519%, which translates to about 80% per month or 14.5% per week.

Onkel Paul