Steve,
I can't speak about the license issue. Someone else will have to
answer about using spanning sync on multiple macs to one Google
account.
However, MobileMe syncs more than just calendar and address book; and
spanning sync doesn't synchronise absolutely everything. For example,
MobileMe will also sync safari bookmarks, ToDos and RSS feeds.
Spanning Sync doesn't seem to transfer birthdays or anniversaries
between the Apple contacts and Google contacts; nor does it get groups
right -- so you'd lose those between your two macs.
Like they say: "horses for courses".
I hope this helps.
Andy