John Deere offers a liquid cooled Yanmar diesel in their mid-line and top of line garden tractors. X595 for sure. I think one of the new x7xx's is diesel and last year there was one in the 300 series.
I have an older 455 which I bought used last year. It is one rugged powerful little tractor. I test drove a Kubota BX2300 last year, and if I hadn't fallen into the deal I did on the 455, I probably would have gone that route. The BX is a great value -- not necessarily a better tractor than the JD, but dollar for dollar you get a lot (4wd, 3pt hitch, and PTO standard -- they're all options on JD).