A very weird but extremely strong backlink..what on earth is that?

I was checking one of my sites today using linkdiagnosis.com, and i found a very strong backlink where linkdiagnosis told me its strength is 500!

(Usually, the average link is anywhere from 0-75 when i examine links, i have never seen such a strong link before and certainly not to any of my sites)

Now, the site in question seems to be a very weird (unknown, foreign) bookmarking site. (At this point i don't want to reveal the URL, please understand)

The link looks like this

http://somesite.somename.xx/go.php?us=ab54aqc8d24c55da

there is no anchor text and linkdiagnosis tells me its a "text link", there is no other outgoing link and the page is PR4.

I examined the page and what it does is simply opens an iframe to my site from their site, but there is a noindex/nofollow meta tag on the page.

This looked really very weird to me, i remember having seen this site once already, probably linking to some other site.

Can anyone make sense of this?

(Edit: It seems to be some foreign "digg" style site i never heard of)