http://en.wikipedia.org/wiki/Sybil_attackShort summary: An attacker could run tens of thousands of Bitcoin clients to isolate certain nodes from the network and then double-spend his coins.
The Freenet project has come to the conclusion that the only proper way to prevent this attack is to let the users explicitly decide who they connect to and encourage them to only establish connections with persons they know instead of strangers from the internet - "Darknet"-mode was born where to establish a connection you exchange public keys with your friends.
For usability it still supports hybrid mode where it prefers your friend-peers and fills up the remaining connection slots with strangers.
Freenet is about anonymity so preventing the sybil attack is crucial. And given that Bitcoin is about money, it seems to have the same importance here.
So IMHO the UI of bitcoin should
(1) Provide the ability to establish permanent connections
(2) Provide robust references to connect to your friend: It shouldn't be "IP:port" because many uses have dynamic IPs. It should rather be some public key hash which is used to query the IP of the node from the network...
(3) Encourage users to use friend connections so by displaying a warning if they have not enough permanent connections which explains the sybil attack