Apple just sent an email out to developers noting that they can begin submitting applications for review. Apple noted at the last press conference that they planned to open the Mac App Store within 90-days and that developers would be able to start submitting applications in November.
According to the 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."
There is still no definitive date for the Mac App Store opening date, but it is nice to know that developers will be able to start submitting apps today.