Good question Forseti. Yes, a bit of background is allowed and Salty's pic fits (my vision of) the brief. As always there is room for interpretation.
By the way, I won't exclude any photos from the poll that I don't think meet the brief - I don't think that's my role and the voters will have the final say in those cases. I will exclude any that weren't taken over the weekend.
Since you asked, I have looked at my own flickr gallery to test my own pics against my own view of the rule.
Here's one where there is a central subject but the background is an important element to the composition so I would say it doesn't qualify:
This one is borderline, probably OK as is, but I might crop a bit off the top to make the point a bit more:
This one has some background but only enough to define the subject, so it's OK:
And this is pretty much spot on:
