to get a PR 7 ,theorically,you should be linked from (at least) a PR 8 page with only an outbound link to you. Considering how that's unprobable,and the fact that PR increases with logarithmic (or similar)criteria,your result have to be considered unreliable.Try the Google toolbar;it will surely return to you a different result.
Stuart...I'm seeing updates with my firefox toolbar but not with the internet explorer download. It looks like they are in the middle of an update. Some of their data centers show an increase in page rank while others are still showing the old results.
Yea I have a blog hosted by blogger and just moved to a PR 4. I have another blog hosted there (was started first) that has a PR 0. I think freshness of content and quality of outbound links are playing a role in this one.