I don't even think this is accurate. Granted, there is something off about how the terms stall out in instant, but for example as soon as I finish typing bittorrent both instant and auto complete kick back in and show as normal: http://img513.imageshack.us/img513/6820/bittorrentac.png
In contrast, if you type out a sex term it quits instant and auto complete and never restarts them, you must hit enter: http://img7.imageshack.us/img7/7349/asexac.png (mostly safe for work - no results)
I imagine the real reason google is doing this is ad placement. After all instant is just about having the ads up longer. Google "bittorrent" and a standard search term like "real estate" - notice any differences? Yup, they don't sell ads on the bittorrent serp (just like they wont sell adsense on a torrent tracker)