Here's a suggestion: once we start hitting the block size limit consistently it will provide users with an incentive to include generous fees in their transactions - this (together with the high volume of transactions per block) might surpass the block reward. But we're a looooooooooong way from there.
By the way has anyone ever noticed there's a correlation between the network deficit (cost of maintaining the network subtracted from price of btc*new btc produced) and the btc price?
http://blockchain.info/charts/network-deficitIf you divide the deficit by the amount of new bitcoins mined per day (3600) you'll pretty much get the spot price on that day: