There are various places to notify people in Lotus Workflow via Mail, e.g. about new activities or via automated activity.
In some cases a notification via Sametime would be a) sufficient and b) a lot faster than mail.
My suggestion is to add a radio button to allow further refinement of the demand:
"Notify by mail only" [default - current behaviour]
"Notify by mail AND Sametime (when person is available online)"
"Notify by sametime only when person is online (send mail as backup when person is offline)"
"Notify by sametime only (and discard notification when person is offline)"