Only a couple days after I voice the expectation, Amazon shows once again they'll never let a competitor do anything alone for long. Just got an email about Preorders now being open to KDP users. I'll definitely be using it for my next release, at the very least to learn the system.
Not sure how much of a bump a low-seller like me will experience from preorders - Smashwords says all preorders will accrue to the sales on the first day, giving a boost to sales rank which in turn increases the likelihood of the book being marketed to other readers as a "maybe you'd like this too!" offer. I imagine the same will be true of the KDP system.
At first glance it seems that Preorders will be available to ALL KDP users, not just Select as I half-expected when I saw the email. Hopefully that's true, and will not change.