There have been numerous studies linking "screen time" (TV, video games, Internet) with childhood obesity.
Generally the explanations given are either that screen time dramatically increases exposure to advertising for high calorie junk for or that it simply takes away from active calorie burning play time.
I think the explanation is likely more subtle - the more time parents feel it's acceptable for children to watch television, the less time they're actively involved in parenting and the less responsibility they may feel towards cultivating a healthy lifestyle in their children.
While my wife and I certainly use the television as a means to try and help during tougher times, we certainly also consciously try to limit our daughters' screen time and instead try to engage them in otherwise more cerebral or physical pursuits.
I came across mention of these screen time "tickets" on a blog entitled Parent Hacks.
Basically the tickets can be distributed on a weekly basis just like an allowance and then they can be exchanged for TV, Internet or video game time.
To find high-resolution, print ready PDFs of these tickets, head over to the blog flagrantdisregard where that kind and obviously computer friendly dad has created these notes worth 30 and 60 minutes in multiple colours.
Should you use them? I don't know, but certainly if you're struggling to figure out a system, these might work well.
If I only gave one piece of advice about television viewing and childhood obesity though it wouldn't be as challenging a piece as limit screen time, it'd be a real simple thing - get TVs out of your kids bedrooms and try to do more stuff outdoors as a family.