[SLL] Anybody know why my Ubuntu sshd server isn't taking my DSA public key?

[Xeno Campanoli]

I'm able to log in to my CentOS server using a DSA public key and no 
password.  It has version:

OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006

but my Ubuntu server, which is Hardy, and has:

OpenSSH_4.7p1 Debian-8ubuntu1.1, OpenSSL 0.9.8g 19 Oct 2007

isn't taking the thing.  In both cases I insert the key to the 
.ssh/authorized_keys file using vim, and for the Ubuntu/Hardy I get 
prompted for a password.  Go figure that.  ???