Harlan Carvey of the Windows-IR blog has finished developement on a utility for determining the OS from a ram dump either dd-style or a VMWare .vmem file.

http://windowsir.blogspot.com/2006/09/os-detection-explained.html

I've read Harlan's book cover to cover and I'm a big fan of his. I would have to guess from some of his other projects, like the Windows Forensic Server is that the focus may have been remote. But also, having a tool provides an automated, accurate, and documented way of collecting this data versus, saying that you knew it was <insert OS here> from the logon splash screen or whatever. I guess there are just too many scenarios to say exactly why they would use it, however it may only be just to see if they could actually do it reliably with the least amount of system interaction possible. For me it makes sense because most of the stuff I do is remote, however if you work in an environment where every machine you get has been unplugged and shipped to you for imaging, then its probably not that useful.