How to release the /boot space.

If you've a lot unused kernels. Remove all but the last kernels with:

sudo apt-get purge linux-image-{3.0.0-12,2.6.3{1-21,2-25,8-{1[012],8}}}

This is shorthand for:

sudo apt-get purge linux-image-3.0.0-12 

linux-image-2.6.31-21 linux-image-2.6.32-25 

linux-image-2.6.38-10 linux-image-2.6.38-11 

linux-image-2.6.38-12 linux-image-2.6.38-8