It looks like Reverse DNS feature is planned but I was not able to find more details on any timeline / planned release date.
You can find out more on those features on Provide Reverse DNS for the Azure Virtual Machines as well as reverse DNS lookup proposals on Windows Azure Feature Voting website.
EDIT As pointed out by @franzo, Windows Azure platform now support reverse DNS records at no additional cost. Reverse DNS support is for all PaaS and IaaS Cloud Services. You can find out more about that feature on Announcing: Reverse DNS for Azure Cloud Services.