Practicality:
- NO2 will inhibit bacterial growth
- It's really expensive
- It is extremely flammable, so you'll have to worry about ventilation.
- Storing a flammable gas under pressure in a non-certified container (your keg) could be risky. You may have to worry about static electric build up and whatnot.
Legality:
Via my grandfather who is a dentist: Nitrous oxide is legal to own under federal law, but state law varies like Brandon mentioned above. Recreational use is most likely a misdemeanor. I'm not sure if the bubbles in the beer being ingested would constitute recreational use. Like you said, they use it in whipped cream all the time.
Side note: The "Force carbonating with something other than CO2" title is a little misleading when you're asking about the legality of using nitrous oxide.