There are only the three priorities, and you can only mark something as one of them (or as having no priority).

How you choose to interpret the priorities, on the other hand, is up to you. You could treat priority 3, for example, as tentative, and mark any tentative task as priority 3.

If that's not flexible enough for you, you could always tag tasks with "tentative".

Hope that helps!

I agree with andrewski that priority 3 would seem to be a good choice if you know for sure that you will actually do the task at some stage - just som details missing. Otherwise, I would recommend you to use priority "4" (no priority) as the most tentative or sketchy type of task, and later to promote those tasks to a more active status after you have finished your current (or higher priority) tasks. I use:

3 = will do as soon as I have the time
2 = am doing now at normal speed
1 = am doing now at rush speed

This works for me. Hope it helps.

What about using the inbox for "tentative"?

Tags and inbox are good ideas. I suppose (now that you've pushed me to think outside the box) that I could make an entire list titled "Tentative" and keep those things which are tentative in there until they become definite.

Per the GTD approach, I use a serparate "someday" list that I can periodically review to consider moving things over to an active list. This keeps them from cluttering my view of active projects and tasks.

A separate list is (indeed) what I use for my "Someday/Maybe" list (with "tentative" items).

Works fine!