It might be a bot testing if your site could be vulnerable to SQL injections and attempt most likely failed.

One of these mysterious bots:

http://securestate.blogspot.com/2009/03/analysis-of-real-world-hacking-attempt.html

http://secshoggoth.blogspot.com/2009/03/odd-sql-injection-attack.html

http://wirewatcher.wordpress.com/2009/11/13/cz32ts-an-interesting-banana/

this is an SQL-Injection Attack (which failed)
someone tried to reade out the database... this could be an direct attack or only an bot which tests several sites.
Moodle escapes the user-input befor passing it to the database, so your system should be secure.

At the moment we (a team of students - FH Gießen-Friedberg - University of Applied Sciences) are developing an Moodle-Intrusion-Detection & -Prevention System using PHP-IDS and Suhosin.
This Plugin can e.g. block users who try to inject-code or send warn mails to the administrator.
You as administrator can also see several statistics about the attempts to break into the system.

At the moment we are in the last phase of developement and commited the idea to moodle-developer.

hardmoodle@lok-soft.net