The dining plan is essentially a pre-paid credit system. Assuming you're talking about the basic plan, you get one table service credit, one quick service credit, and one snack credit per night of your stay (you cannot have the
DDP for only part of your stay). The cost of the dining plan is $45.99 (including tax) per adult per night (everyone in the room must be on the same plan). So for the two of you, it would cost $91.98 per night. You must be staying on-site to purchase the dining plan. You also must have a vacation package (room plus tickets) to purchase the DDP. If you have an AP then you can do a ticketless package.
A table service credit entitles you to an entree, dessert, and non-alcoholic/non-specialty drink per credit. Tax is also included, but not tip. The quick service credit is fairly similiar, except it's only good at quick service locations.
Disney has a pretty good brochure that describes the plan and the locations where the credits can be used:
http://disneyworld.disney.go.com/me...alog/WaltDisneyWorld/en_us/PDF/2010Dining.pdf
Essentially, to know if it would worthwhile, you really have to decide if you think you're going to eat about $92 per day in food for one sit down, one quick service (aka counter service), and one snack. A good way to get a first cut at this is to look at the menus of the places you're eating at on allears.net or wdwinfo.com and see how much it would cost if you were paying out of pocket (OOP).
My rule of thumb is that if you aren't planning on having enough sit down meals to use up those credits, then the DDP is probably not worthwhile.
I think that you have to add the DDP to your reservation before you arrive at WDW. I can't remember exactly, but I think it might even need to be added at least seven days in advance (not sure about this), but you might want to check and see how long you have to make this decision.
I saw on the resort board your post about getting a room during President's weekend. This made me wonder if you have any advance dining reservations (ADRs) already made. The ADR window opens up 180-days in advance and many of the most popular restaurants fill up fairly quickly. There is always the chance of a walk-up, but during the busy times, the wait is longer and there is a greater probability that the most popular places will not be taking walk ups. There will be somewhere to eat, it just might not be your first or second choice - especially if you were planning on eating in one of the parks or at a character meal.
