[mckoss]

You're confusing "clients" (or Bitcoin nodes) with miners. All clients share information about pending transactions through a peer-to-peer network. A small % of those clients are also mining - incorporating (or not) transactions into a block and attempting to find a sufficiently rare hash value.

Since most mining is done in pools, there is quite a concentration of power in pool operators; if they want to ignore certain types of transactions, they can just omit them from the blocks that they mine.

The real threat is that if any mining pool that had more than 50% of the world-wide hashing power, they could effectively control which transactions would become verified, and which would remain forever "pending". They would accomplish this by ignoring all other mined blocks, knowing that they will be able to outrace the rest of the world in producing the chain representing the "most difficulty".

Ordinary (non-mining) clients can disrupt the propagation of transactions around the network - but as long as you have a possible communication path to SOME miner, you should be able to have your transactions registered in a new block; like the Internet, the bitcoin network has redundant paths to foil censorship from a minority of nodes.