Quite often in my training sessions I encounter the scenario where it would be nice to rerun a task that's just be executed as an on-demand task. However, the Task History only allows for re-running tasks that were executed as Scheduled Tasks.
The way to handle this is to develop the habit of creating any task that you might need to re-run at a later time as a scheduled task. Scheduled tasks can be created as a "Run-Once" task for far into the future, but this is not particularly necessary. Once a task is scheduled, and appears in the Scheduled Tasks listing, you can mark the task as DISABLED, which permanently removes it from the schedule. (A Run-Once configuration with a date far in the future, e.g. 12/31/2099 is still probably a good safeguard).
Then use the "Run Task Now" option from Scheduled Tasks to execute the task on-demand, which will also make it re-runnable from the Task History list. In addition, some task features can be modified from the task Properties in the Scheduled Tasks list, such as the computers a task is targeted to.
If you have questions about this technique, post them in this thread, and I'll be happy to answer them.
Subsequent to this original post, we added an additional feature to the product that allows you to explicitly save an on-demand task to the Scheduled Tasks node, without have to actually schedule it. On the Scheduling and Notification Options dialog, check the "Save Task.." checkbox under the "Run Task Now" radio button.