Last month evidence surfaced in the form of a Geekbench report, showing that Apple is indeed working on a 13-inch variant of the new Retina MacBook Pro. Well, as pointed out by MacRumors, a second MacBookPro10,2 (same identifier used in the previous Geekbench finding) entry has appeared in Geekbench results database.
There are some minor differences in this new entry when compared to the older one. First, the new machine is sporting the latest build of OS X 10.8.1, which was just seeded to developers last week. This new entry also shows a machine equipped with 8GB of RAM, whereas the previous entry was only equipped with 4GB. Lastly, the BIOS string was updated as recently as July 18th. The previous entry was updated on June 11th.
As for the rest of details, they seem to match up nicely with the previous entry. They are both equipped with a 2.9 GHz Core i7-3520M (same as current high-end 13-inch MacBook Pro), the same motherboard identifier, and both share a very similar Geekbench score (around 7800).
Some reports have pegged the release of a 13-inch Retina MacBook Pro sometime in October. This does not sound unreasonable to me. It gives Apple time to manufacture the machines, while also giving consumers time to purchase the device for the holidays. There seems to be no technical reason for Apple to delay the launch, but rather a marketing one.