Maybe with 10.6 they're trying to reduce the development cycle back to shorter like it used to be. It's wise with a complicated piece of software to do regular releases. Also there has been some discontent with 10.5, more so than past releases, so maybe they just want to put it behind them.
As for code names, Microsoft has stopped using code names and just uses the numbers by themselves now, e.g. Windows 7. Maybe Apple will not come up with a replacement for the cat names, but rather just drop code names altogether.
There are still 3 names to use, as Apple have trademarked them, no one knows which order though. There is still Cougar, Lion & Lynx