Dan,
At the risk of getting further into design (sorry Don), I wouldn't remove the call completely. It's pretty useful default functionality to get an email when you've been assigned a task.
I'm not sure what the best way would be to preserve this: it would be great if the return value of the callback could be used to determine whether it had sent the message, but I don't know if that's a bad convention, or how well supported it would be across platforms.
FWIW, in another comment I saw in the code, Lars (?) suggested that notification messages would ideally be made from Tcl. This is simpler for a couple of reasons, but mainly that the Tcl code can construct an HTML notification message with subsite URLs easier. The HTML that you send out in the email will likely be reused in the workflow status pages (or vice versa).
