As a server myself (not at Disney, though, I wish), I would just like to say that buffet or not, you should still tip the same. We have a salad bar where I work and many people will just eat that and believe that doesn't warrant a tip.
However, most servers don't just get to keep what they make. A LOT of restaurants insist that their servers tip people out, aka. the hosts, bartenders, and sometimes food runners. This tip-out is based on the server's sales and a buffet counts just as much as a full-course meal. If they are already giving 5% tip-out and someone only leaves 5%, they basically worked for free.
In addition, most servers are paid minimum wage or LESS per hour. I, for instance, work in PA, which only pays its servers $2 an hour (after taxes we are left with about 50 cents!). I have never found a state that pays its servers over minimum wage. I feel that if the server is working, they should be paid and that pay comes from people's tips.
I am pleased to read the posts above where people tip 20%. I can assure you that many people DO NOT. Some do believe in tipping less, even when the server works really hard. Granted there are days when the server DOESN'T warrant it (even I've left little or no gratuity before), but the server really has to have NOT earned that tip.
So thank you to those of you who do tip 15-20% for adaquate service and above that for those that receive spectacular. This should be especially true at Disney because you are experiencing the Disney magic and this is one way to spread that magic along to others. And even when you return home to your local restaurant, you can still spread some magic and put a smile on the face of a hard-working server who is doing her best to please you!
Now, if you'll get me get off my soap-box, I will climb down and wish everyone a magical evening. Thanks for letting me vent after another rough day at my restaurant. Believe me - my Disney vacation cannot come fast enough!!