Now that several Controls support promptText Property it would be nice if this could have a distinctive look so not to confuse it with an actual input.

There are several standards on the web to "mark" promptText as such.
They can be divided in two categories:
1) old-fashioned "text enhancement": surround prompt with some distinguishing "brackets" (usually either two dashes "-- prompt --" or two brackets "[[ prompt ]]")
2) new-look via graphic enhancement (grayed text, alpha-blending, bluish border, inner-shadow, italics, ...).

There might be some default for this, but I think it would be better to simply add a css pseudo-class and let User chose his way; something like:

.combobox:prompt { ... }

I do not know if there's a way to implement (1) via css (even given the right pseudo-class).

There should also be a way to define a common style for all promptText for all supporting Controls (css class).