You always have to open any application to work on it. The push version would simply notify you when an incoming call was coming in, and open the app accordingly. Yes, there will be a delay to open the app and connect but I doubt it would be more than the latency on existing cellular calls.

The real pain comes when trying to replace the existing background app: ipod. Being able to listen to streaming music is worthless unless you can send an email whilst doing so. Also, GPS apps take a while to get a lock and this would be another example of where backgrounding is useful.

It isn't redundant simply because it's located elsewhere, it is redundant because your files are duplicated elsewhere.

Here's the caveat I was trying to point out (sorry, from the article it sounded like a 1:1 ratio but it isn't): since your "usefulness" is really dependant on how much time you spend online, that is how the credit is calculated. time(%) multiplied by space allocated = credit. So if I allocate 100GB of my disk and I'm online 50% of the time, it counts as 50GB. This makes sense, as now my 100GB of data needs to be duplicated at least once to remain online.
(see http://www.wua.la/en/support/faq/c/16#id001601 )

The problem is you are now depending on the average time online over a period of ____ hours/days to calculate this number. So if I have 100% online time, and I disconnect, your fragment of data is now gone (rendering X number of files incomplete, depending on the fragmentation algorithm).

Is it me, or does that not add up?

Suppose each user wants to keep 100GB of data, and they share 100GB of their own HDDs. Where is the space for redundancy?