(Post Updated 2/8) Thanks to Mara123 for linking us to the first images of Rupert on the set of Half-Blood Prince at Gloucester Cathedral! Filming has been taking place there since yesterday and is expected to continue until tomorrow.
The article states:
"Reporter Kate Nicholls managed to get a quick look behind the scenes - and saw crew busy making magical potions ready for the filming of the sixth book in the series - Harry Potter and the Half Blood Prince."
"Two sets of crutches were also spotted arriving at the set - leading to speculation the scenes after Ron falls from his broom in a game of quidditch could be being filmed. "