We're still working on the problem of synchronizing data online/offline, but the storage issue looms large. Will users be comfortable having all of their information stored online? I think so, but RIA developers need to take pains to ensure that the data is secure and trust is not misplaced.
The above topic in general, i.e. whether or not to keep a copy of your data, or synchronize your data online is becomming a bigger and bigger question and issue. This is something I'm very interested in, and doing some work on myself.
I don't believe it's a simple issue of all your data online or not. I do however want ubiquitous access to my data, but most solutions so far come up very short. Mainly this has to do with security and privacy. I already use del.icio.us, Flickr, Backpack, Basecamp, Gmail, and various other tools that store some of my data online, and or hold a copy of some of that data. But, I most surely do not keep my Quicken files online, or various other documents and information that I consider particularly sensitive or needing explicit security.
But, I really want this. I want all my data available on any computer I use. I'm starting to look at things like S3 to create my own solutions. One notion is to store everything on the server encrypted. This works, but becomes a significant overhead when you start storing large amounts of data, or large files like media. Maybe it is an option per file or per resource location.
I think it will be very interesting to see what Google and folks do in regard to this aspect. Sure, GDisk would be great and all, but only for some things, at least until there's some level of security. And believe me, just having some code of conduct, terms of service, and employee guidelines for those working at the data centers, doesn't mean there can't be abuse - you need to make it essentially impossible for folks to see your data (when you care about security/privacy of said data).