Thanks. I don't need to go back that far but I'm not sure that really matters in this case because with Modernizr, I will be creating styles on elements that use boxshadow (using rgba) and different ones for those not supporting boxshadow.
So if they don't support boxshadow, I won't use rgba in this case. There are separate checks for support of boxshadow and rgba. I think this probably leans more to the standalone use of rgba rather than using rgba in boxshadow. So I get the feeling it is probably pretty safe to assume that if they support boxshadow, they also support rgba.