I think this is true for the first month or so after the Pro SC:BW players switch, but most of them are in high masters after the second or third month. (I base this on a discussion in State of the Game that took place this week or last.)
I watched Hyun's first tourney when he made the switch last year. (I think it was Hyun, anyway.) He went into the touney with only a month of play time, and won the first game decisively. The next two he lost, but it seemed to me that the loss was due to not having a full appreciation of the Queen and the importance of injecting. His opponent (not sure who that was now) recognized that and modified his play in the second and third games to emphasize this weakness. This was a weekly tourney on ESL, not one of the big events.
I acknowledge that that's only one match and one pro, however I listen to a couple weekly SC2 casts and, when the discussion comes up, the consensus is that current BW pros real strength is in their APM and perfect build order execution. Both of those do translate to SC2 pretty well, and usually result in a pretty quick rise to the top. (I should clarify, it's the memorization and execution of a build order that translates, not the order itself.)
This only applies to Korean pros. I don't know if the BW scene was large enough anywhere else for there to be enough pros to make any generalizations.
--jesse