What kernel version comes with Ubuntu 10.04.1?

The Linux kernel in Ubuntu 10.04 is currently upstream version 2.6.32.21 with many Ubuntu specific additions. You can find this yourself on a 10.04 machine by reading the /proc/version_signature file, and reading the version string without the "-" in it:

cat /proc/version_signature 
Ubuntu 2.6.32-25.45-generic 2.6.32.21+drm33.7

If you don't have a lucid installation available, you can also see what versions are available in each release by going to https://launchpad.net/ubuntu/+source/linux and scrolling to the bottom; the latest kernel in each release pocket will be listed. Thus (as of 2009-11-02), the current updated kernel for lucid is 2.6.32-25.45 and there's a kernel version 2.6.32-26.46 proposed to be published as an update undergoing testing in the lucid-proposed pocket.