I have written a post earlier about why you should keep your Joomla sites updated for safety reasons.
Phil Taylor published this Tweet today:
A lot of people getting old versions of #joomla 1.5 hacked today - been fixing sites all day for customers...
UPGRADE NOW to #Joomla (latest version)
I couldn't say it better myself. It's crucial that you upgrade to the latest version of Joomla.