A big enough mining pool could DoS some or all users by consistently producing a longer block chain than anyone else's (which clients would accept as authoritative) while omitting any transactions they don't wish to let through.
This is true. A 51% attack in which the attacker prevents all transactions from going through is probably bitcoin's largest vulnerability.
However, even at today's difficulty levels, this attack would be very expensive to maintain. With the introduction of dedicated mining ASIC's coming soon, the difficulty will rise substantially, which will offer even more security against this type of attack.
On the other hand, even with those ASICs, the government could buy more of them than anyone else. And then make us pay for them. I don't think the threat is worth discounting.
