Not soon enough, but I'd imagine it'd be the same timeline as the EVO 4G. I think the OG black came out in June and the white came out in July as a BB exclusive and everywhere else in August or something like that. So in short...about 1 to 2 months.
Actually, I think the design changes they've made (for better or worse) will look better in white. Especially with the red stripe/kickstand running down the middle of the back. I think it will also subdue the contrast with the polished aluminum around the sides.
Actually seeing in white make me want it more than in black. Hope they announce a white version before pre-orders start. IMO, its looks better that way. Why can't they just come out with both colors at launch?