Preview at WWDC likely in June, followed by September launch.
Apple Mistakenly Sending Out OS X Server Replacement Up-to-Date Codes
While many of these replacement codes are already coming through, users today have begun reporting that Apple is mistakenly sending out codes for the OS X Server components of Mountain Lion rather than Mountain Lion itself.
I just got my second code and was able to redeem it. The only problem is when it redeemed it came up with an error saying OSX Server cannot be downloaded because it requires OSX 10.8.0 to run. It seems they sent me a redemption code for server instead of mountain lion. The pdf with the code clearly says that the code is for mountain lion...At least half a dozen users have reported the same problem, with the issue presumably affecting a much greater number of users receiving codes today. One user who called Apple about the issue was advised to file another request through Apple's firstname.lastname@example.org support mechanism.
My purchased tab only shows OSX Server - no mountain lion...
OS X Server is also available through the Up-to-Date program, but is limited to users who purchased server models of the company's Mac Pro and Mac mini lines.