Okay: sitemeter is the tracker I use to track visitor numbers etc., so it's nothing sinister.
However, given it's popped up on Tumblr, I can only assume that there's something temporarily amiss with the Sitemeter site as - obviously - I have sweet FA to do with Tumblr.
We'll give it 24hrs. If it's still happening, I'll drop them an email. For the record tho, I've not received anything (OSX / Chrome).