BTW - It's shift then DnD the mail to the mini-cal to create a task with that date.
But just realized it uses that for a start date but the end date is set to the current day.
Thus you get a "can't have end date after start date" on close.
Bug 29221 - Tasks - don't require end date Bug 30088 - tasks : mini-cal drag should use different start/end date logic