The context for those of you who didn’t catch it is that the kids have written an episode with Matt Smith’s Doctor on his own encountering an old foe and Albert Einstein. Since he’s on his own, it’s presumably after the last episode (and is set to air after The Wedding Of River Song, so that would make sense). The fact that he’s wearing a fez in it would also indicate that there are no companions around to forcibly remove it.
The Doctor is clearly pleased to be reunited with his Egyptian headwear, and remarks that he missed the Fez following River Song’s pre-reboot destruction of said silly attire.
“I missed you! I can’t believe River blew you up!”
You can see for yourself in Doctor Who Confidential, which is currently on the iPlayer (the line in question occurs around the 35:50 mark).