A full analysis of the 2008 dining plan has been posted to the site here.. Here’s a quote from the summary:
Although not as good a deal as in years past, the 2008 dining plan can save families money under certain circumstances. The plan makes sense if you intend to have one sit-down meal per day and you’re able to get advance reservations for your first- or second-choice restaurants. Our advice is to first book advance dining reservations (up to 180 days before your trip) at your selected restaurants, before committing to the dining plan.
With the new changes to the plan, we’re pretty sure that it rarely makes sense to use two dining plan credits at Disney “signature” restaurants. The analysis above includes five new meal-by-meal and item-by-item breakdowns, showing what you can expect to save or lose using the plan.