Journey's End and, as Orionhunter said, in a way El Pescador, have plans which are nearly all-inclusive. Captain Morgan's, too. And Cayo Espanto as well. But these are not quite true all-inclusives in the sense of including everything including drinks, tours, transfers, activities, tips etc. in one price.
Mopan River Resort in Benque Viejo (Cayo) is a true "net net" all-inclusive. Everything included from transfers from Belize City, tips, drinks, meals, canoes, tours including those to Tikal, plus very nice lodging in deluxe cabanas, are included for around US$115 per person per day (at some hotels you'd pay that just for the tours.) I'm not usually a fan of all-inclusives, but for some people this is really a great deal.
Another all-inclusive is opening near Hopkins in early 2001: Kanantik Reef & Jungle Resort. It will offer everything inclusive, including diving. But rates will be steep: US$300 per person per day.
Some of the jungle lodges, including Chan Chich, Chau Hiix, duPlooy's, Chaa Creek and Jaguar Paw, offer more or less all-inclusive packages. But most are not really all-inclusive because things like transfers or tips or booze may not be included.
Belize First Magazine http://www.turq.com/belizefirst/