They will ship with Mavericks
I would like this to be the case however I don't think they will. Its purely down to logistics in the production cycle. We have to assume that if the Haswell MBP is available to ship this week then production must have started some time ago for them to build enough stock. Added to this is the shipping time to get the units to the distributors and shops ready for the rush and you end up with a reasonable lead time.
GM builds of Mavericks are available but unless it was ready prior to production of the MBPs then I cant see how they would easily get the new OS on there without unpacking, installing and then repackaging a lot of machines.
I am expecting a release with the current version with a free upgrade to the new one at release.
Personally I hope you are right and I am wrong though as it would be nice to have the new OS on their straight away on launch day