I know it is not good practice to assign due dates to tasks when using the GTD methodology. However, when serving multiple clients where there are many items attached to a specific due date, I find it better to assign the due dates in the task rather than put it on my calendar (which clutters my calendar far too much, especially on my Smart Phone). Also, when you assign due dates to Tasks in Outlook you can have them show up on your desktop client calendar at the bottom.
The negative to this method is that you may have many items that are past due that you wish to move to another date. My Google-Fu must have been busted, because I could not find out how to do this, so I resorted to manually changing each one.
Today, I finally figured it out! And being the nice guy I am, the fruits of my labor are now available to all. 
Step 1: Choose “Due Date” as the current view
When you do this all of your tasks will now be grouped by due date.
Step 2: Make sure the target date exists
If there are no tasks with the due date where you want to move your tasks, then you will need to create a dummy task with the due date you need.
Step 3: Select
Using Ctrl or Shift to select all of the tasks that you wish to update the due date.
Step 4: Drag and Drop
Drag and drop those tasks into the group with the due date you need.
Magically, Outlook will change all of the due dates to be the due date of the group you dropped the tasks into.
Is there an easier way? Please let me know in the comments.
Thanks for sharing. I’ve been trying to figure this out for awhile and this works great.
Hallelujah! I have a persistent problem synching my Blackberry Tour to Outlook since I started using the NextAction! 2.0 as an alternative to Blackberry’s rather weak Tasks application. Specifically, Outlook Task due dates randomly reset to dates in the distant future. This technique works to gang-reset Task dates via Outlook, which is a big help: I like NextAction!, but I think it’s the likely culprit here. Now I don’t have to abandon NextAction!, which would have been too bad, as it is a big improvement on Blackberry Tasks for GTD adherents.
Interesting. I tried it but didn’t like it. Too much trouble to create the dummy task. But it DID get me thinking…. and here’s what I came up with.
I tiled two Outlook windows.
On the left I’ve got a Calendar View (Week)with the Navigation Pane and ToDo Bar minimized and the Tasks due showing below.
On the right I’ve got my ToDo View list with Navigation Pane and ToDo Bar minimized. I tweaked the Active Tasks view to group by date.
Now if I’ve got too many Tasks due on any given date, I just select them on the right and drag them to a new day on the Tasks on the left.
Franklin-Covey can suck it! I made a better mouse trap! LOL
Thanks so much! Seems like such an obvious answer to someone who is used to moving batches of things in Excel, but I couldn’t figure out how to select the darn items with no checkbox. I too much prefer to keep these things in a task list instead of the calendar — too much visual clutter is overwhelming & counterproductive. And I’ve wasted so much time moving tasks one at a time…this made my day!
Very helpful workaround – thanks!
What I can’t figure out is why Microsoft provides a menu button for edit task due date in the T-Do bar and then grays it out so you can’t use it.