Won't Dropbox do that for you? Will IOS let Dropbox sync all of your pictures to Dropbox....then sync it back down to every device that has the client installed? It is cross platform too.....
It very well might, haven't dug deep into it's functionality but I believe it will take more manual work and require separate photo libraries, as I don't think photos will remain synced between both Dropbox and iPhoto. It also means I have to set a shared photo folder in Dropbox and each client that wants to share must then have their own Dropbox account. Not as slick as what will likely be inputting an Apple ID and flipping a switch.
I realize it's not a benefit to everyone, especially those not as embed in the Apple ecosystem, but it's a welcome change for me.