Hi,
Purely on personal experience (don't know if it applies universally) I think you are correct.
Our Australian site is site.com/order/ and has never had this problem. Our UK site is order.site.com and is the only site where we have noticed this occuring (i.e. sub-domain being treated as an entirely separate domain)
Google also ahs some good resources on this:
http://code.google.com/apis/analytics/docs/concepts/gaConceptsDomains.html
This also might be useful if you want to adjust the analytics code as opposed to the site structure
http://code.google.com/apis/analytics/docs/tracking/gaTrackingSite.html#domainSubDomains
Dylan