Apple sent an email to developers notifying that they can begin submitting applications to Mac App Store for review.
According to email, "The Mac App Store will be opening soon. You can get ready by submitting your Mac apps for review now. Log in to the Mac Dev Center for details on setting up an iTunes Connect account, creating Apple-issued Mac Distribution Certificates, and preparing your apps for the review process."
Apple will begin testing their new Mac App Store on Mac OS X Snow Leopard, and will come featured as part of Lion next year. Apple still hasn't announced when the store will open, but users will be able to beta test it in the near future.
You can watch how Mac App Store will work in the video from Back to the Mac: