Yup... it's a whale (a northern bottle nose to be precise), and it's actually in the Thames, swimming past London's Houses of Parliament.
The complete story here. More pictures here.
Only in this instantaneous age of the internet would this story, and the pictures to go with it, spread so fast.
Here's some excellent background info. on the species.
By the way... if you just happened to be in London today and snapped some pics of this marvelous marine mammal yourself, you can send them here:
yourpics@bbc.co.uk