As someone who fell for it I'll tell you how. In the Volume Licensing portal Microsoft chose to list Windows 7 Pro filed under 'Windows', just as all versions of Windows Vista (inc. Enterprise) had been.
However, for some unknown reason they decided to list Windows 7 Enterprise under the category 'Software Assurance Benefits' several pages in, IIRC. So early adopters of Windows 7 were left assuming that maybe they had re-jigged which editions a Select Agreement customer was entitled to. After all, the Volume Licensing portal was unlikely to be wrong, right? And you could clearly see that Vista had been laid out sensibly.
So I installed 150 Windows 7 Pro workstations by accident too. Luckily, they're all being replaced this summer anyway. While I was researching (in vain) how to in-place upgrade them I found a post by some poor guy who had accidentally rolled out over 1,000 Pro machines at a university. Poor bloke
