Men who spent more than 23 hours a week watching TV and sitting in their cars (as passengers or as drivers) had a 64 percent greater chance of dying from heart disease than those who sat for 11 hours a week or less.
Many of these men also exercised regularly.
I'm disposed to believe that remaining in any position for too long is bad for a person: our bodies aren't built for it. However, the crux of the problem doesn't appear to be sitting as such, but the associated lack of movement for most of us while we're seated. (It would be interesting to see if users of old-fashioned, non-motorized wheelchairs had the same probability of dying from heart disease as the seated-by-choice.)
Only time will tell whether further research will confirm or refute this initial conclusion. In the meantime ... well, it can't hurt to sit a little (or a lot) less, unless you've got bunions....