Not sure if anyone still checks these comments but I know of another fic with this pairing. The thing is that it seems to have been written before we knew Ian's first name so he is called Rick Edgerton in the fic but besides that its an awesome fic called Unexpected:
http://www.thetenthmuse1.madbrilliant.com/numb3rs/numb3rs_main.html

I absolutely love this pairing [as I love Ian/Don] and this fic was a blast. I'm here now because I had to come back and reread it. I don't suppose you can be convinced to do another fic of this pairing or just Don & Ian???