Like Primecoin? 
Quote from: http://bitcoinmagazine.com/5635/primecoin-the-cryptocurrency-whose-mining-is-actually-useful/
Self-adjusting block reward – Bitcoin is known for its controlled currency supply algorithm, which guarantees that only 21 million bitcoins will ever be generated, as well as specifying the rate at which these bitcoins will come out. Primecoin follows a different path. The number of primecoins (XPM) released per block is always equal to 999 divided by the square of the difficulty, a formula which should converge to some maximum if the difficulty increases linearly. Given that Moore’s Law states that computing power increases exponentially, and the effort it takes to find a prime chain is exponential in its length, that is quite likely to hold true.
It's similiar to PPcoin and Neocoin (Scarcity enforced by Moore's Law, mint rate gradually drops as difficulty rises).
