A reader (Thanks Jim!) mentioned earlier today that his SSH logs were showing access attempts utilising elements of the reverse DNS name of the IP address being accessed.  For example using  isc.sans.org results in the userids isc, sans and org. This may be cause a number of hosting providers use the domain name itself as the userid for shell access for customers.  In light of the breach at dreamhost earlier this week http://blog.dreamhost.com/2012/01/21/security-update/ this may be what is going on. 

If you are noticing the same in your logs and you can share some log lines please send some in as I'd be interested in taking a peek.

Mark H