As we know DW is still filming and SJA has started filming as well - but they're being very secretive. Apparently Tennant was at the read through for SJA which obviously indicates he's going to be in one of the episodes, but thats about as far as we know.
On the 20th April there was some filming in Penarth which included Tennant and the TARDIS. It might have included other things but the security was very very tight.
Credit to these wonderful photos goes to Mali :)
Now this could have just been for Doctor Who (they were filming there the previous week) but Tennant isn't wearing the same clothes as he is for the last special, which can only suggest that this is him in an episode of SJA!
A HUGE thanks to Guy and Mali :)