The photos are mostly on buses or on mainline train journeys but you can easily see how this would work for London Underground journeys.
The photographer's choices for this are fun, but I loved a shot that I blogged about a few years ago where Meg Pickard captured a lady reading on the Tube. This wasn't set up but just a lovely spontaneous moment where everything worked together brilliantly
Photo by Meg Pickard
Chutney Bannister's "The Surreal Line" set of photos on the Tube also show how cross platform ads can mingle with commuters to create some interesting perspectives.
Thanks to @MichalD for initially alerting me the to current "photo bombing" photos and you can see the full set of them all on Shortlist.