Synergy Building Company has built quite a few sealed crawlspaces. Generally, whenever we are building a new home, or putting on a new addition, we strongly recommend that the crawlspace be sealed and semi-conditioned space. This is also the case with sealing attic spaces. The advantage of keeping HVAC equipment and ducts in a semi-conditioned space makes sense for a lot of reasons. Now, we are finding more interest in sealing existing crawlspaces and attics, as part of an attempt to establish more energy efficient homes.
Advanced Energy, in Raleigh, is a great resource for learning about the comparative advantages of various energy efficient techniques in residential construction. Here is an interesting article on sealing attics for those who are considering this upgrade to their home.
Here is a picture of what sprayfoam insulation looks like in a recent Synergy project.