This issue will be (is?) solved with the "Client IP information in DNS requests " DNS extension[1]. A year ago, David Ulevitch (OpenDNS's owner) mentions in HN post that he already got it working for all Google properties and few other CDNs (except Akamai).[2]
My guess is that they're not receiving strong demand from their major customers - since they have so many massive users and a long-term code-base it makes sense that they'd be rather conservative.
[1]: http://tools.ietf.org/html/draft-vandergaast-edns-client-ip-...
[2]: http://news.ycombinator.com/item?id=2941948