I would think they will have to mention it. I guess the obvious time for him to get out was when Rose looked into the heart of the TARDIS.
Here's an interview with a typically hyper John Barrowman.

The only spoiler is about Mr Saxon.
[...]
“It’s weird being back on the show,” Barrowman says, not looking remotely fazed. “It’s a completely different Doctor and assistant to last time. Jack’s looking at the Doctor going, ‘Oooh, this Doctor is funny and quirky and sexy. The other one was like a U-boat captain – a bit dull.’ So there’s an edge there that wasn’t there before. And there’s a lot of stuff resolved in this series for Jack. There’s this whole thing of where Jack comes from, what his story is, and when I read it, I rang Russell and said, ‘I don’t know what you’ve been smoking, but that is brilliant!’ God, I wish I could tell you what happens!”
For reasons too complicated to explain, I do actually – despite all the BBC’s admirable, warlike levels of secrecy – know what will happen in the season finale of Doctor Who. “You do?” Barrowman yelps. “Ohmy-God, isn’t it AMAAAAAAZING?”
Barrowman is semi-incoherent with enthusiasm. We bounce up and down on the sofa, screaming, and then do a high-five.
“But I am a fan,” Barrowman says, when he’s stopped bouncing with joy. “Like any British subject, I used to watch the show as a kid, hiding behind the sofa. And when we moved to America when I was 8, I watched it on PBS on Sunday night, consequently failed every spelling test on a Monday morning. This is why I’m such a s*** speller. It’s all down to Doctor Who.”
He beams again, and then slaps the table with his hand. “I’m a man with a little boy inside him, who’s living his dream!”
Later on he mentions that he once auditioned for the role of James Bond, and was told to come back in ten years’ time.
“That was five years ago, but before I came out,” he explains. “An American audience might just about stomach a blond Bond [Daniel Craig] but not a gay Bond. But you know what? Russell T. Davies has written a complete action hero that’s iconic, and kids love him, and mums, and dads, and gay men and women. And I’m completely content.”