According to Wikipedia, this is the list of superstaitons:
Out of those, I've only ever heard of WTBS and WGN. The others might just be regional superstations. TBS just announced today that they are going HD, so that would just leave WGN, which is technically a different channel than WGN9-TV-DT in Chicago, running a different schedule (due to SyndEx and CW affiliation). As for incentive, there's always cable, and, since it's a different channel that only runs on cable, I don't see any reason to stop doing a 4:3 SD channel in 2009.