W: There is no public key available for the following key IDs:

lennyへのupgrade時にaptitude updateで下記メッセージが表示された。

# aptitude update
(中略)
W: There is no public key available for the following key IDs:
4D270D06F42584E6
W: You may want to run apt-get update to correct these problems

http://akazaru.blogspot.com/2009/02/apt-get.html
を参考に

apt-get install debian-archive-keyring

で解決。