Tom,
I think it is important to define your use case. First and foremost is what you are trying to achieve self-service "consumer" backups or is it "provider" backups?
Provider backups are somwhat easier as it doesn't require you to expose that functionality to the end user. It is somewhat similar to what you do today with vSphere but with the additional burden of backing up metadata associated to the vApp (these metadata aren't available in the traditional vSphere backups). We do have guidances on how you back those up (via vCD APIs). Data Recovery doesn't yet interface with vCD to do that but I know many backup vendors have new releases of their products that are supposed to do that. Note that for file level backup and even VM level backup you may not need this level of integration (since you are not changing the structure of the vApp which is still there). You are essentially replacing vSphere corrupted/missing pieces (Guest OS files or VMDK files).
Self-service consumer backup is a bit more tricky and it would require you to give end-users a parallel (multitenant) interface / tool that can do that (since the vCD UI isn't customizable as of today and you can't expose additional non-built in services). EMC has demonstrated in the past an example of such tools: http://virtualgeek.typepad.com/virtual_geek/2011/08/tech-preview-avamar-vcloud-protector.html
Note in that page a doc for how to do "provider backups".
Massimo.
Massimo Re Ferre'
VMware vCloud Architect
twitter.com/mreferre
www.it20.info