I'm new to Android and was just wondering, if I wanted to develop something like HTC's Sense UI, what is the "entry point". I've gone through the developer docs and it seems those docs only refer to developing applications, but not replacing the Android default L&F.
What's a good starting point?
That depends on what you mean by "the Android default L&F".
HTC Sense is mostly a replacement home screen, plus a few extra apps and such. There is a sample replacement home screen in the sample code that ships with the SDK.
If you want deeper changes than that, you will have to build your own firmware.
HomeScreen.. look in sample apps in the sdk for an idea at least the starting point..