Ocelots spawn using the hostile mob spawning system. This causes the following behaviours:

• They spawn only a certain distance to the players (like hostile mobs)
• They quickly despawn if you are leaving the area
• They don't spawn on peaceful making in impossible to gets cats in peaceful mode.
• They don't spawn on servers with spawn-monsters=false in the server.properties making it impossible to get cats on such servers (without cheating spawn eggs).

Neither of these behaviours make really sense as ocelots are (currently) not hostile. Ocelots should spawn using the same method, wolves, cows, etc. spawn so they can still be found in peaceful mode. The current behaviour irritates a lot of users who are wondering why they find no ocelots while playing on a peaceful map. (google "no ocelot spawning minecraft" or similar for examples)