Forget about Serendipity. A visit to their forums and a bit of Googling reveals that the code is sloppy at best and dangerous at worst, it is plagued with comment spam (and no good way to combat it as WordPress and MovableType have).
But I’ve found another little gem of a program called PivotLog. There’s a testbed install here. There is no (easy) way to skin the site or switch stylesheets, but you can — as you can in WP 1.3 Alpha, change the design from the backend by simply uploading different design files into Pivot’s template directory.
The other thing that is almost a deal-breaker is that there’s no way to directly import a WordPress Blog. So what I’ve had to do is export WordPress to MT and then import the MT data into Pivot. I haven’t finished that yet. I want to do so and then play around with it for a week or two before I make the final switch.
Yes, I’ve found it THAT intriguing.