public key warning

[Mark Grieveson <dg135@torfree.net>]

Hello.  After a recent upgrade of my Etch, I get the following warning:

Fetched 30.2kB in 9s (3350B/s)
Reading package lists... Done
W: There are no public key available for the following key IDs:
A70DAF536070D3A1
W: You may want to run apt-get update to correct these problems

What does this mean?  Should I be concerned?  Is anyone else receiving 
this warning?

Thanks,

Mark