This has been touched upon in various threads, I'd like to discuss it in more detail.
Transaction fees are currently a negligible fraction of the miner's income, it's all about the block reward. There are presently 7200 coins scooped up every day, soon to be halved to 3600. Assuming most miners will not want to mine at a loss in the long term, and assuming they expect a reasonable period of return on investment of say one year, they will only purchase up to 3600*365 = 1.3 million coins worth of equipment and electricity. This is the maximum value of mining equipment in the world, assuming mostly rational decisions are made. Anyone wanting to stage a disruption of the network (aka >50% "attack") will need to spend roughly that much on their mining equipment and electricity. This is the best-case scenario, where the attacker relies on best publicly available mining technology, instead of investing into development of their own technology.
All the talk about ASICs making the network more secure is irrelevant: whatever currently available best technology is (GPUs, FPGA, ASICs,
) - the attacker needs to spend an equivalent of roughly one or two million coins to stage an attack (or more precisely, a disruption).
One caveat is that some miners might be
investing gambling based on expectations of future value of coins, so their equipment and electricity would cost somewhat more in fiat, but I'd be surprised if this plays a big role.
What do you think?