Please be aware, we are seeing an increase in Joomla 1.5 malware, sql injections, and security breaches. We understand it is not always feasible to do so, but we do recommend upgrading your Joomla.
Using tools such as injection monitors and firewalls can also help prevent these malicious and destructive codes.
We will continue to help where we can in identifying these issues via our support ticket system.
Can you give us more information about what you're seeing? Is the weakness in Joomla 1.5 itself or is the weakness in the DT Register version running on J1.5?
How would I know if I am vulnerable or if I'm under attack?
Also, make sure your JCE Editor extension is up to date, this had a bad security issue back in early 2013 (January I think) so this one needs to be updated as well.
Otherwise I've got a lot of clients running Joomla 1.5 sites without issue.