It looks like OEMs (original equipment manufacturers) will receive Windows 8.1 on September 1st, 2013, that is if you believe a Hungarian Windows website which posts the following:
Windows 8.1 - get on 1 September, the OEMs?
A leaked e-mail to 
Microsoft's OEM partners and system builders sent out yet as of yesterday evening, which will let them know that the OEM system pricing is 
from 1 September take place, so now it was sure to be a partner with the 
Windows 8.1 - 's.
Link Removed
The exact date is not known, of course, only one date is available at the beginning of September. In return, the day before surfaced in 
Windows 8.1 Build numbers, which were as follows: 
6.3.9600.16384. winblue_rtm. 130822-2140 ; 
6.3.9600.16384.130821-1623 - you can see that the building has received both the 
9600 's and the 
Delta number, so you might be clear to us that their 
candidate RTM , RTM that is selected from the group belonged versions. This means that Microsoft has chosen them out of the final 
Windows 8.1 builds. From the above, as stated earlier the Russian sources, therefore we learned, we can easily imagine that after August 21 to three days to the Microsoft to finalize the Windows 8.1 was, for 
72 hours in the period, which the Russian news source reported . therefore still can not be sure that the complete update package for the Windows 8 PC, Windows 8.1 final.
Source: 
http://oprend.hu/news.php?readmore=1149