From that link:
Estimate Strategy
Extrapolating bitcoin difficulty or price is pure voodoo. It is much easier to predict the relationship of the two parameters in form of the Mining Factor. The Mining Factor 100 is the value in USD of the bitcoins you can generate if you let a 100MHash/s miner run for 24 hours. If the Mining Factor 100 rises above $2 or so everybody buys mining equipment and thus increases difficulty. If it falls people will stop mining eventually. The estimate starts with the current Mining Factor and decreases it exponentially such that the decrease accounts for the factor decline per year. Please note that a profit/loss by holding the coins is not accounted for in this estimate.
Things to consider that might eat into your profit:
The values above are only a snapshot. The network and markets are moving quickly. Check out these diagrams to get a feeling for it.Looks like if your mining operation is not profitable now, it probably will not be in the future.
With rising bitcoin exchange rates it might be more profitable to buy bitcoins than to mine. There are spreadsheets available in this thread or this one (with some FPGA data) for a more custom calculation.Bitcoin exchanges: MtGox, Tradehill, CryptoXchange.