Most likely to be able to keep up with the initial demand. Instead of guessing how many white vs. black models folks were going to buy, the initial blind production run buy is all black. Once the "I have to buy one in the first 3 days or I'll die" folks have bought them then can move on to trying to figure out how to split up the production distribution.
It also helps delay some folks a couple of weeks. (if extremely found of white option will wait. ) More iPhone sales later doesn't hurt as long as the customer is frozen in place.
I'm not sure I buy that logic.
Why would apple want to slow down sales?
2yrs ago I ended up getting a White 16GB 3G, because they had sold out of the 16GB Black 3G's at the Apple store I was at. I liked it instantly even though I was a bit dispappointed with the thought while waiting in line. This same thing would happen with this model. There are some people that would settle for the color to have the capacity.
Personally, I'd be a bit disappointed if they didn't have the White model off the bat. I'm not sure how I feel about the white front, and I really want to see both of them in hand to decide. (Only thing holding me back from preordering)