Then on November 19th, 2008 (or shortly thereafter), I noticed this:
Nov 19 15:04:22 rosalita sshd[40232]: error: PAM: authentication error for illegal user alias from s514.nxs.nl Nov 19 15:07:32 rosalita sshd[40239]: error: PAM: authentication error for illegal user alias from c90678d3.static.spo.virtua.com.br Nov 19 15:10:20 rosalita sshd[40247]: error: PAM: authentication error for illegal user alias from 207-47-162-126.prna.static.sasknet.sk.ca Nov 19 15:13:46 rosalita sshd[40268]: error: PAM: authentication error for illegal user alias from 125-236-218-109.adsl.xtra.co.nz Nov 19 15:16:29 rosalita sshd[40275]: error: PAM: authentication error for illegal user alias from 200.93.147.114 Nov 19 15:19:12 rosalita sshd[40279]: error: PAM: authentication error for illegal user alias from 62.225.15.82 Nov 19 15:22:29 rosalita sshd[40298]: error: PAM: authentication error for illegal user alias from 121.33.199.39 Nov 19 15:25:14 rosalita sshd[40305]: error: PAM: authentication error for illegal user alias from 130.red-80-37-213.staticip.rima-tde.net Nov 19 15:28:23 rosalita sshd[40309]: error: PAM: authentication error for illegal user alias from 70-46-140-187.orl.fdn.com Nov 19 15:31:17 rosalita sshd[40316]: error: PAM: authentication error for illegal user alias from gate-dialog-simet.jgora.dialog.net.pl Nov 19 15:34:18 rosalita sshd[40334]: error: PAM: authentication error for illegal user alias from 80.51.31.84 Nov 19 15:37:23 rosalita sshd[40342]: error: PAM: authentication error for illegal user alias from 82.207.104.34 Nov 19 15:40:20 rosalita sshd[40350]: error: PAM: authentication error for illegal user alias from 70-46-140-187.orl.fdn.com Nov 19 15:43:39 rosalita sshd[40354]: error: PAM: authentication error for illegal user alias from 200.20.187.222 Nov 19 15:46:41 rosalita sshd[40374]: error: PAM: authentication error for illegal user amanda from 58.196.4.2 Nov 19 15:49:31 rosalita sshd[40378]: error: PAM: authentication error for illegal user amanda from host116-164.dissent.birch.net Nov 19 15:55:47 rosalita sshd[40408]: error: PAM: authentication error for illegal user amanda from robert71.lnk.telstra.net Nov 19 15:59:08 rosalita sshd[40412]: error: PAM: authentication error for illegal user amanda from static-71-166-159-177.washdc.east.verizon.net
... and so on, the alphabetic progression went on and on.
Several hosts try to access the same user, up to minutes apart. When any one host comes back it's more likely than not several user names later. The full sequence (it stopped December 30th), is available here.