True
7264 is an escrow build. The last working RC key version that was stable was 7260.
Escrow builds are builds that have been submitted for final (RTM) release to manufacturing approval.
The way it works is MS gathers a build number they think will make RTM . It is then put before a variety of test and gets input from all those working on the project. If nobody has any open issues or finds a reason why it should not be the final then that becomes the RTM, until it gets final approval it is escrow.
So beta keys will not work because it is not a beta. Also be aware that using it is prosecutable by MS if they chose to do so.
I would just get a copy of 7260. It is stable, very close to retail and doesn't expire till march, 2010
7264 has also been rumored to be the Final build, but again it will not work with the RC serial.
x86:
7264.0.090622-1900_x86fre_client_en-us_Retail_Ultimate-GRMCULXFRER_EN_DVD.iso
Size:
CRC32: 0B916606
MD5: 44F04E9E5762714C3D75A3C9C88DE251
SHA1: 59BA011913B00A820A1E002B9BEDEF139AE15B07
x64:
7264.0.090622-1900_x64fre_client_en-us_Retail_Ultimate-GRMCULXFRER_EN_DVD.iso
Size:3.00GB
CRC32: 05F87B79
MD5: 670818DB149170C652414E1F75E9390C
SHA1: C6FE386E97944511820291F515A8CBB3DDA329A6
Was not the final, but an important build nevertheless . It has been sent to over 10 OEM for product testing.