Is there a uniform measure of "difficulty", that when multiplied by price, will give "profitability of one hour of mining"? I recently open Bitcoin Pie, a site that compares crypto-currencies.

Given the exchange rate in BTC, I would like to add another column that tells "relative mining profitability", regardless of the hardware used.

So, if at a given moment, BTC mines at difficulty X, NMC at Y, and FooCoin at Z, with the conversion rates to BTC being 1, N_BTC, and F_BTC respectively, just comparing X, Y*N_BTC, and Z*F_BTC should give an indication of which one a rational miner should mine.

What is a good number to use in the difficulty column? Where are the APIs for it? Is it just plain "difficulty", or is it some other number?

I'm not sure how to take merged Namecoin mining into account here. Any ideas?

CPU and GPU coins should not be measured on the same scale.
The standard formula to determine the average profit per day for a given MH/s miner on a given blockchain is as follows:

(S/(D*4295))*60*60*24*R*P

Where S is the speed in MH/s rating of the miner, D is the difficulty rating, R is the reward (50 BTC/block for Bitcoin) and P is the price in USD of a single coin.

That I know of there is no standard profitability measure but it would be easy enough to create one given standard values of S. If for example we define the standard as S=100 MH/s then we could easily compare the profitability of mining various blockchains in the resultant USD calculation.