There are several already but I know of at least two who were updated to 4.0 only but after a week were updated once again so that they included 3.1.3 again.
I think allot of developers will realize how important it is to included 3.1.3 for a long time still since 3.1.3 is the last version that the original iPhone will ever see.
Last estimate that I heard was that 25% of those original iPhones sold in that first year are still being sold today. One of those in app analytics companies released some info about that several months ago. That's allot of original iPhones in use today. Mine is still activated.