According to this New Yorker article (
http://www.newyorker.com/reporting/2009 ... ntPage=all), DFW spent time at a halfway house in Brighton (a section of Boston) in late-1989-1990, following his breakdown while a graduate student in philosophy at Harvard. That's where he met a lot of people in treatment, attended AA sessions, and -- eventually -- started working on IJ. As the novel makes clear, the addiction/recovery themes are pretty central, so this makes a lot of sense. Also, helps explain how a guy like DFW, with both feet in privileged intellectual society, would be able to get such a good bead on addicts from different walks of life (as he said to a friend at the time, "everyone here has a tattoo or a criminal record or both" -- another trait that shows up among Ennet House residents).
The article is a good read in many ways, especially though on the addiction/mental health issues. One thing I found surprising is that DFW's main "substance of abuse" was marijuana, and it was that he sought treatment for -- just like Hal. Not inexperienced with that substance myself, I was struck by how severe Hal's "withdrawal" symptoms were -- suggesting, on some levels, that something else (DMZ?) might have been going on there. But after reading about DFW's experiences with pot & withdrawal therefrom, I'm inclined to believe he was writing from experience about serious psychological consequences from quitting that relatively "soft" drug (see also Kate Gompert, of course, who may be an even closer DFW-analogue, as someone with both mental health and substance abuse issues).
edit: also, the chronology sketched by DFW's family & friends in the New Yorker article fits precisely the description in the anonymous letter, so I'd judge that very well substantiated, if not 100% proven.