I think we're going to have live with the fact that we won't get a straight answer other than this: Our purchase of the Dining Plan includes all gratuities. The Dining Plan brochure doesn't mention anything about "18%". That's inside information and as such we'll have to live with it as a rumor, nothing more or less.
If I remember correctly, the number 18% came from an explanation of why the Dining Plan increased in price between 2005 and 2006. However, this is no in any way part of what guests are promised.
Also, we've learned from servers from around the country that it is standard practice for gratuities to be split among your entire service staff, not just the one waiter you give the money to. A waiter is assisted by food runners and folks who clean the tables, as well as the bartender, and the waiter is obligated to tip them all out -- share the gratuity with all of them. Again, that's a nationwide practice in many restaurants, and Disney is no different, so if any server actually kept the 18% all to him/herself, it would be wrong.
We do know that the non-Disney restaurant get an allocation, and we have heard rumors that at least some of them figure the gratuity on that allocation from Disney. That's up to those restaurants, of course, but it might be a firm basis to avoid the non-Disney restaurants, since there are such indications that they're not as generous with their staff as Disney is.