With Adobe CS3 you only have to author once (from HD source) and then change one setting to produce the SD output version. Rendering two versions takes longer but can be done unattended on one computer while continuing other tasks on another, which is how I worked in SD too.

Regarding the comment someone made about tape costs, for me that's identical with HDV compared to DV since I use the same tapes, so no change there. Other than initial HD upgrade expenses, my main cost differential between an HD project and SD ones is ~$8 for a blank BD-R disc and a few extra cents worth of electricity for longer rendering times, so that's trivial compared to my total fee for a project. Recovering my upgrade costs is the main reason I need to charge anything extra for working in HD, and once that's paid off I could reduce the upgrade fee to a more modest amount and still come out ahead.

As I've noted before, one reason to switch to HD now is so you have a better chance of recovering your upgrade costs while customers are still willing to pay extra for HD production and your competitors are all charging accordingly. Once enough of us have HD gear which is paid for and have sorted out our workflows to take HD into account, the average upgrade fee may drop and that would make things harder for people who upgrade then. Something to think about...