We (voxel.net, AS 29791) offer dual-stack on all server and cloud products. As others have pointed out, SoftLayer is an excellent example of a hosting provider that Gets It on a large scale.
Sadly, v6 support on popular "cloud-only" services is suspiciously absent. Terremark vCoudExpress, Savvis, Amazon EC2, among others don't support it today, or on any public roadmaps... -a
