i'm posting this in case it helps anyone :-)
i am unable to use the Upgrade version of Windows 7 Professional to upgrade my Boot Camp installation of Windows Vista Business.
there is a reproducible problem which aborts the install at an early stage.
judging from the number of blog posts which report successful installs of Windows 7, i have to assume that they are using the Full Install disc, and not the Upgrade disc.
(i have verified that the particular Upgrade disc i have, proceeds beyond the stage where the reproducible problem occurs, on a regular Windows PC)
[update: have a look at this Apple Support Document]