For every generation of Android system, we could find out one corresponding kernel version.
The main effort of development is not only Android system but also need to take kernel part.
For example, the next generation of Android 2.3 we call Gingerbread use Linux kernel 2.6.35.
The developer will get the BSP(Board Support Package) from platform provider (ex: Qualcomm).
But the BSP will be later than Google's release about few month.
Platform provider, such as Nvidia and Qualcomm, porting Linux kernel 2.6.35 to their platform and also do some little change on Android release.
Before system developer get the official release of BSP, they could do some earlier job on it.
The existing version of Android 2.2, Froyo, use Linux kernel 2.6.32.
The possible porting schedule before BSP release might be porting Gingerbread AOSP(Android Open Source Project) with kernel 2.6.32,
and harder one is porting Linux 2.6.35 on platform with Froyo because kernel is hardware-dependent.
Finally, try to merge 2.6.35 & Pure Gingerbread release from Google.
The purpose of these jobs is shorter the TTM (time to market).
