HTC lately made the kernel source code available for a few selective smartphones and now for the HTC Flyer tablet.
Google Android operating system is based on the Linux kernel which is an open source code, but not all the code inside a HTC devices is totally free. Thats why HTC plans in the near future to release a software developer kit for the Sense UI that is running on-top of the Android for all HTC handhelds. The release of the kernel source code is ment for developers that would want to develope new apps to fit the Sense UI which is running on any HTC.
If you are a developer you can grab the kernerl source code from here (the HTC developer site).