Android Kernel Versions

2.6.25
The original (version 1.0) of Android for the G1/ADP1 used Linux version 2.6.25

2.6.27
The 1.5 release of Android (cupcake) for the G1/ADP1 used Linux version 2.6.27

2.6.29
As of September, 2009, the kernel/common.git tree for Android has a 2.6.29 kernel. Whether this means that the Donut release will include this version of the kernel remains to be seen.

Selecting a kernel tree for a new port
This article has good information on porting Android to a new device. Note that it debates which kernel tree is appropriate to start with, for porting to a new device:
 * http://www.linuxfordevices.com/c/a/Linux-For-Devices-Articles/Porting-Android-to-a-new-device/