Talked to my dealer and he said that the 4231TD and 4226D are available through the end of 2012 and the 4500 will get a 25hp Kubota D902 diesel engine. So they do have a diesel option.
And before anyone complains about it being only 25hp, have fun finding many manufacturers in the next years producing one higher than that in the 25-50hp range without paying an extra $4k-$6k for the unit.
It is the way it is and in a year or two I am sure most everyone will learn to deal with it. If you want higher horsepower you'll have to go with gas in most cases.
With EFI and DFI there are more fuel efficient gas options now if fuel consumption is a concern.
As far as longevity, I've heard much more positive things about some of the newer gas engines out there than some of the diesels as far as total hours put on the engines and the lower maintenance, but I don't have any personal experience besides mine and being an AC unit it isn't one to compare with LC for longevity even though it has been fantastic.