Is there a quick keyboard or mouse shortcut to insert your signature in a reply email in Outlook 2003 SP3?
It doesn't look like there is one. Here's Microsofts own list over the shortcuts that are available.
Here's a blog post about how to create your own shortcut that might help you.
The process I used (thanks to Par for the good link):
Note that it appears to not be possible (without an addin) to make a keyboard shortcut for your signature in Outlook 2010.