yum removing old kernels?

[Danny Ciarniello]

When I do a yum update I get

Installing:
 kernel                  x86_64     2.6.16-1.2080_FC5  
updates            16 M
...
Removing:
 kernel                  x86_64     2.6.15-1.2054_FC5  
installed          81 M

Since when does yum remove old kernels automatically?  Seems a bit 
dangerous to me.  What if there's a problem and one needs a fallback?

Dan.