Let me preface this with saying "in the realm of wedding photography"...

There is a difference between the 'lift expectancy' (MTBF) and the 'usefull life expectancy' of a camera - especially at the proffesional, high-end level.
For instance, your D2H will still produce pictures just as good as it did years ago... but it's likely your clients won't be as happy with 4mpx output now as they were when the camera was new. That having been said, I believe that 12mpx or 16mpx will 'suffice' for much much longer than 4mpx did.

So, assuming you want to stay at the high-end of clients, which is generally going to require the high-end of image quality, which is generally going to require you to produce files somewhat comensurate to then-current technology. Assuming the above logic holds... how long could one expect to go without upgrading cameras?

Specifically, if I want to run a pair of D3's with a D300 as tertiary backup.. should I budget to replace them in 2.5-3 years (one generation), or 5-6 (two generations)? I'm kind of of the persuation that image quality with the last iteration of Nikon pro gear has gotten to the point that I won't need to replace for 5-6 years. (Canon appears to have hit this level about 2 years ago)
If there is a 2-3 year replacement in order, perhaps it doesn't quite make that much business sense for me to outlay 12k (amortized to 4-6k per year) on bodies alone and three D300's would be the better bet. Bear in mind, I don't currently have a large set of clients to help me get jobs via refferals, and thus don't get many jobs as of yet.

What about lighting gear - how long can you expect flashes to last? What about lenses - I know that can last decades, but with much quicker advancements, how long until you'd notice a large improvement via upgrading?

The reason I ask this in 'Forms, Filing, and Functional stuff' is because I'm looking at it from a projected profit/loss/expenses angle... not a "camera x is better than camera y, neener neener" angle.

Thanks for your assistance...