DOE/Steven Winter Assoc has recently completed a study and built standards for spray foam insulating duct in attics. I attended a webinar where the author, Bill Zoeller presented their findings. It was fantastic:
Early adopters will need to be significantly involved in the process to insure quality, probably having to lead the contractors until these strategies and best practices for implementation trickle to the field.
You can encapsulate unburried duct. It doesn't meet Challenge Home requirements so you wouldn't design this way, but apparently is a significant improvement for retrofit:
Here's my spray foam picture album for those interested.