What happens if a monopoly grows in Bitcoin mining and payment processing? What happens if a monopoly grows in Bitcoin mining and payment processing?  Here is a scenario that I have been thinking about. Some big corporation, like the Bank of Amerika, decides that Bitcoin is the future and starts to buy thousands of graphic cards. At the same time the payout for solving a block decreases and many small Bitcoin miners figure the cost of electricity and time is not worth it anymore. But the big corporation is subsidized by the present financial system and shifts profits from banking services into paying for Bitcoin processing until only it and a handful of other large banks control the majority of the Bitcoin network.  What happens then?  Do they start raising fees and requiring a credit card number to process a transaction?
In that scenario, the corporation could:


Launch a 51% attack at a whim
Pretty much control the Bitcoin economy, driving the prices up or down how it sees fit (at least until it spends all of their Bitcoins)
Control which transactions get included in the block or not. This could require one to use their specific client and thus gain more control over the network
Have a big impact on the Difficulty, as it could turn its machines on and off
Control the rate at Blocks are mined (for example, holding onto a solved block for certain amount of time before broadcasting it)
Control who gets to see the most recent Blocks. With this, they could hold out a long chain of solved Blocks for their own client, and when a standard client would finally mine a block, they could just release a couple old blocks to invalidate that chain


All in all, if you own majority of the hashing power, you can pretty much dictate your own rules, up to what the Protocol allows.