Was disappointed in all these "constant network hashpower/exchange rate" mining profitability calculators.
So, just made the opposite one. Customizable. MS Excel based. Offline. BTC<C.
Hope you will find it helpful too.

Some screens:
http://belarusminsk.s3.amazonaws.com/BitcoinCalcScreen.png

http://belarusminsk.s3.amazonaws.com/LitecoinCalcScreen.png

WWW: http://belarusminsk.s3.amazonaws.com/MiningCalc.xlsx

Torrent: http://belarusminsk.s3.amazonaws.com/MiningCalc.xlsx?torrent

Input your estimations of Network Hashpower and Exchange Rate for 50+ steps in time (=up to 2 years for 2 weeks interval), and get Break-even period, Profit and ROI for three kinds of mining hardware of your choice.

Cheers.

#update 28 June 2013
Published version v0.2-130628:
* '356 days in a year' typo was fixed. Now it is 365  
* One more time step was added. Now there are 54 time steps in total.
* Months count in the H column is starting from '0', but not from '1'
* Cells A30-35, A47-52 and A64-69 are hardcoded to be just numbers, but not expressions. There is no need for expressions.