I've recently been asked about our tool vendor, Omniture, and their support for IPv6. It seems they currently don't support it, but are working towards it.
Does anyone know the current proportion of mainstream traffic coming through IPv6?
There's a bunch of data you'd lose with people coming from IPv6 endpoints. Most critically would be any Geo-IP mapping you're doing. If you're using IP addresses for visitor identification (and please don't!) you'll also have problems.