The Sybil attack is described here:
- http://en.bitcoin.it/wiki/Thin_Client_Security
Also, if you can cut communications with 90% of the miners' hashing capacity, you only need 10% of mining capacity to perform a 51% attack.
A miner that finds access to pools blocked can mine solo (e.g., using P2Pool even) or go on Tor, but the network is more resilient with more full blockchain-verifying nodes.
Now the current database that was used for the Bitcoin.org client ended up being a poor choice, and a switch to a better choice (which didn't even exist when the Bitcoin client was first architected) is being developed as we speak.