I only vaguely remember this fic, but from what I remember:
1) Miles brought in Cobb in the original dreamsharing experiments
2) Arthur was (one of?) the first test subjects when very young
3) He was kept in chair with permanent hookups to his brain (I think)
4) Cobb allowed Arthur to escape one day....I think around when the project was being terminated
Eventually, Arthur found Cobb again, and started working for him. He continued to use the hookups from the experiments to do research, I think on the internet.
Finding this story is driving me crazy, I hope it sounds familiar?
EDIT: FOUND!!!
We Used to Wait by
chibi-lurrel