Y'all probably want this:
https://github.com/bitcoin/bitcoin/pull/491However, I don't think you can fix all the problems that a fixed transaction fee cause; the real problem is that basic economics says that you need to let the price of a scare resource change, ideally in a market, to match the underlying real costs.
(bitcoin's fee structure isn't right either, and fixing it to create a market between miners and clients is high on the TODO list)