Oh, this is lovely :) And, as a multishipper whose preferences include Doctor/Rose/Jack and Doctor/Jack, I don't even see it as crack. It's tender and poignant and beautiful, and I'm happy that both Jack and the Doctor have each other to care about and be there for company. And that they'll never forget Rose. :)

I wonder, was this the Doctor finally letting go of Rose, too? Since s/he did manage to call out Jack's name at one point?

(As for Ianto, they're not serious. Ianto even said so - 'dabbling', 'it's not what you think'. No reason why Jack should worry about leaving him for the Doctor! *grin* )