To explain why this is meaningful, I'll first give a quick primer on the document.domain property:
The document.domain is by default set to the hostname which is used to access a site.
The document.domain property is not read-only. It can be truncated by however many levels you like, so a site on sub2.sub1.domain.tld could set the document.domain property to sub1.domain.tld or domain.tld or just tld
Finaly, there is an additional check whereby if the document.domain property has not been modified then a page where the property has been modified cannot communicate with it.
Firefox and IE do have this check, but it seems a bit more relaxed. If the upper level domain reads the document.location property this check is seemingly ignored.
Now, one might be tempted to shrug this off, but many tracking scripts, and Google Analytics tracking code references the document.location property, and so any site which runs the Google Analytics code is vulnerable to having lower level domains communicate with the unwittingly.
 The Firefox 3 nightly build does not allow anyone to set the property to tld, as per two patches from trev/Wladimir Palant: http://sla.ckers.org/forum/read.php?13,10863