My experience is with Palms (never tried it on the BlackBerry, though I could, if you're interested) and the Palm browser (Blazer, I think?), and my view is that prosilver and subsilver2, oriented as they are toward a visual experience, do not lend themselves well to handheld browsing. So, I think, you first have to offer a handheld-oriented theme regardless of the handheld.
That doesn't actually answer your question, but I can only give an opinion there based on visitor stats I have access to. My thought is that since handheld/cell phone users currently represent a relatively small percentage of total visits, if one is going to consider them important at all, then being selective within that market is likely going to minimize the benefits of considering that market in the first place.