I name a bike after the first few rides based on what he/she feels like to ride and what his or her personality is like. The name usually just comes to me, and it's usually a human name. My current bikes are:

Rebeccah - my custom steel touring bike. Rides like a smooth dream. She goes by Becky if she's feeling saucy.

Suzie - my cross-country mountain bike. A bubbly girl who likes it fun and dirty.

Mack - my fixie. Named after Mack the Knife. He and I cruise for chicks downtown.

Poz - an old banger hybrid I keep around for people to borrow. Old and cantankerous, but with a sense of humour.