[StarBuG]

The canned replies would be stored in the db for every user that has the right to store canned replies (defined by ACP if possible by usergroup and also number of canned replies that can be stored)

It should be a drop down menu in the WYSIWYG Editor where you see the short titles of a canned reply and if you select it, it appears in the editor box so that it can be posted.
If possible this should work in posts as well as in private messages.

A canned reply should consist of:

A short title (like "Welcome 1", "Warning Spam", "RTFM", etc)
The canned reply itself

The input editor could be in the UserCP or the ModCP/ACP (cause Mods and Admins will be the most likely to use this anyway, but that would limit the options a little)

Cool would be a mouse over effect that shows the whole reply if you hover the mouse over the short title in the drop down, if that is possible.

StarBuG