I'm not interested in working on the same file in two different
versions concurrently, but you pose an interesting hypothetical.

Aside from the issue of different versions, I doubt Win XP will allow
a second grab at a file -- won't it give a sharing violation? In an
enterprise environment, with a large network and many users it may be
possible, but for a single user I doubt it.

There is a format incompatibility to contend with also. 2007 uses a
different format for its files, and although it is backward compatible
-- i.e., 2007 can read all earlier version formats -- the reverse
isn't true. There is a compatibility setting I've heard that lets you
save in older formats.