I too have been looking for a cloud-based photo manager. It is indeed too bad iPhoto/iCloud can't serve this purpose (yet?). Photo Stream is deceptive in that is lures you into thinking your data is secure in the cloud, when its only there for a 30 day window.
I currently have my photo on RAID 1 NAS, meaning the *two* drives need to fail to loose my data, but off-site backup would be better. That said, I'm unwilling to go through the contortions that Terrence above desbribes.
I've been dabbling with Adobe Revel and so far, so good.
Pros:
+ Unlimited photos, perportedly at full resolution
+ Synching with OS X and iOS devices
Cons:
- Costs money, currently $6 a month
- Limited platform support, if you need Android or Windows (Adobe says its coming)
- Very limited feature set just now, it really needs tagging/search, facial recognization, print ordering, etc.
Anyhow, another one to consider.