@Vaccano: So, you are saying that Microsoft should double (Probably more than that when you take into account all the other factors. Really, check out those references that vesuvius posted) their costs to do what amounts to subsidising a small-ish number of developers?
It's simple. If your customers are running XP, then you develop for XP. I really doubt that there are any insurmountable bugs that prevent you from using .NET 4.