I can confirm that in my case it was due to a corrupted download of the ISO file. The size of my file was not correct; and it gave the error mentioned in the question above, here on my VMware vSphere 5.0.
Use a download manager (e.g. Orbit Downloader) and compare the SHA-1 hash of the downloaded file with the SHA-1 value published at Microsofts download page. (Install HashTab, right-click the iso file and click the tab File Hashes) Or download from msdn.microsoft.com, as they use a buildin download manager which checks the integrity of the file.
Both Windows Server 8 Beta and Windows 8 Beta seems to run without any problems at my VMware vSphere 5.0 here.