Looks like you are correct. I found this well-written article on InfoWorld:
http://weblog.infoworld.com/gripeline/archives/2007/05/motherboard_rep.html
Of particular interest is this paragraph:
"Fortunately, I was able to find a Microsoft official who was gracious
enough to look into the reader's questions for me. "The rule is in place to
protect the OEM, or in this specific case the System Builder, so that as
computers are upgraded, the System Builder is not obligated (per the EULA)
to support a version of Windows that may be on what is essentially a new
PC," wrote Tom Moran, director of customer and partner experience for
Microsoft Operations. "Generally, you may upgrade or replace all of the
hardware components on your customer's computer and the end user may
maintain the license for the original Microsoft OEM operating system
software, with the exception of an upgrade or replacement of the
motherboard. Upgrading the motherboard essentially results in a new
computer, to which the original operating system software cannot be
transferred. This is not the case if the motherboard is replaced (same
make/model) due to a defect."
Personally, as someone who has been building PCs for many years, I wasn't
aware of this and I find this disturbing.
Shouldn't "not enough RAM capacity on the motherboard to run Vista" be
considered a "defect"? ;-)
Tim