Yes, a kernel is never deleted automatically. This is the behavior you want!

It is save to deleted older kernels you are not using, but be careful!

In a terminal type the following to find out what kernel you are running.

cat /proc/version

Output should look something like this:
Linux version 2.6.15-28-686 (buildd@terranova) (gcc version 4.0.3 (Ubuntu
4.0.3-1ubuntu5))
#1 SMP PREEMPT Wed Jul 18 22:57:30 UTC 2007

You do NOT want to remove the kernel you are running!

In my case, I do not want to delete anything with "2.6.15-28"

Then you can use Synaptic to search for older kernels with your main kernel
search string "2.6.15".

For example in my case, any kernel image/restricted/source files with
2.6.15.26, and 2.6.15.27 can be safely removed.

The removal scripts will edit the grub menu for you.