Annie Mole's, webmaster of Going Underground, daily web log (blog).
If you like this you'll LURVE One Stop Short of Barking, THE fun and informative BOOK about travelling on the London Underground.
In addition to the one pictured above for Hyde Park, there are ones at Piccadilly Circus & Charing Cross for Horseguards Parade, where I believe beach volleyball will be taking place. There's also one at Wembley Central, just in case tourists might be confused of the stop to use for Wembley (there is a little clue in the name - but some people may need extra help!)
I haven't seen in these on any other lines yet, but perhaps they will be appearing on other lines too. Let me know if you see any.
"The magic, mystery & sometimes maddening shortcomings of London's Tube are documented with love, enthusiasm & sometimes despair by its unofficial social historian." The Guardian