http://www.lifespanfitness.com/workplace/treadmill-desks/tr1...
Hands down, the TR1200-DT3 is the best treadmill desk out there right now. Their bluetooth app is crap, but as a treadmill actually designed for office use, this is perfect. Read this amazon review for ways to customize the programming to be less annoying:
http://www.amazon.com/review/R3C358XW4LPBS7
LifeSpan also has a more heavy-duty model, the TR5000-DT3, $2000. If you're buying for an office setting where multiple people will be using it for more than 6 hours/day, get this one.
http://www.lifespanfitness.com/workplace/treadmill-desks/tr5...
iMovR will have a new unit coming out, the ThermoTread GT Desk Treadmill, that MAY be just as good or better, but you won't get it until the end of the year at the earliest.
http://www.imovr.com/imovr-thermotread-gt-office-treadmill-d...
That's kind of the state of the art in treadmill desks right now. I imagine there will be more options in a year or two, but the LifeSpan models are by far the best options today.