The fees paid to miners are growing exponentially - currently a 10 fold increase in usd every 6 months:
http://blockchain.info/charts/transaction-fees-usd?timespan=all&showDataPoints=false&daysAverageString=1&show_header=true&scale=1&address=
At some point , long before the block reward makes it not worth the effort, the fees will make it worth the effort.
Edit: actually its 100 fold every 6 months on this chart.