As the Bitcoin difficulty increases, Bitcoin miners will stand to get less pay-per-shares (PPS) from BTC Guild. BTC Guild pays a fixed rate per share based on the current Bitcoin difficulty.

Current PPS rate at BTC Guild is 0.00002168093424286754. Current Bitcoin difficulty at 2190865 on 16 Aug 2012.

BTC Guild is a true pay-per-share pool. Every valid share submitted is instantly credited at the current PPS rate, and available for withdrawal [withdrawals must be > 0.10 BTC or > 1.0 NMC].

Meaning of the Bitcoin difficulty term: https://en.bitcoin.it/wiki/Difficulty

What is “difficulty”? Difficulty is a measure of how difficult it is to find a new block compared to the easiest it can ever be.
How often does the difficulty change? Every 2016 blocks.

See below and you will notice the BTC Guild PPS rate is lower after 5 days.