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:

http://bit.ly/sprayfoamducts

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.

Name:  Fullscreen%u00252520capture%252011152012%2520114153%2520AM.bmp.jpg
Views: 365
Size:  57.9 KB Name:  Fullscreen%u00252520capture%252011152012%2520114020%2520AM.bmp.jpg
Views: 354
Size:  60.8 KB Name:  Fullscreen%u00252520capture%252011152012%2520112827%2520AM.jpg
Views: 357
Size:  55.7 KB Name:  Fullscreen%u00252520capture%252011152012%2520112231%2520AM.jpg
Views: 345
Size:  53.7 KB

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:

Name:  Fullscreen%u00252520capture%252011152012%2520112413%2520AM.jpg
Views: 365
Size:  56.7 KB

Here's my spray foam picture album for those interested.