Well, I missed the first five minutes of this episode, so I don't know how Bernard Cribbins got out of the car, but the rest of the episode was quite tense and would have worked quite nicely as a series finale, so I hope they can top it with the real finale.
Which will probably be Daleks. Again.
Fascinating final scene too, with a TARDIS which doesn't seem to want Martha to leave, and the Doctor's Severed Hand (the ship is huge, so why is he keeping it in the console room?) going mental, which usually only happens when there's a Time Lord around. Hmmm...
I didn't pick up any of the "code words" this time, although there were echoes of the first episode of the series as the Sontarans too wanted to turn Earth into a nursery planet. We also briefly saw Rose on the TARDIS monitors, but that's not exactly a clue, since it's quite obvious that she will play a large role in the series' over-arching story.
There was also a nice nod to the Gas Mask Zombies in the episode, and a confirmation that the Brigadier was still knocking about (in Peru, apparently). Neither are probably clues, but they were welcome bits of fan service nonetheless.