Lyle McDonald talks a lot about diet breaks, and their usefulness, in his Guide to Flexible Dieting.

Generally it's a good idea, no matter what kind of diet you're on, to take occasional diet breaks of 7-10 days--2 weeks is better--where you eat at maintenance calories, normal protein, carbs at least 100g daily. This will help upregulate all the hormones that were suppressed due to dieting, and thus help your diet "work better" when you return to it.

HTH!