What is "Warning: /boot/System.map-XXXX does not match kernel data"

[Julian Stoev <stoev@iccl.snu.ac.kr>]

Hi!
I started getting this first on SPARC Debian. And now I get it on
Intel.
On SPARC it was first with 2.2.17
Now on SPARC I get 

{iommu_unlockarea} {___f_mmu_unlockarea}
Warning: /boot/System.map-2.2.18pre21 does not match kernel data.

On Intel I get 
{module_list} {module_list_R__ver_module_list}
Warning: /boot/System.map-2.2.18 does not match kernel data.

Can somebody explain this? Is this a security problem? I remember
reading something about fake kernel modules?  

--JS