I predict that a client for casual users that stores as little of the chain as possible will be in widespread use at least one full year before the size of the block chain database file grows to the size that would take up 1% of the average (mean) hard drive size of typical home PCs.
Forget home PCs for a second. For bitcoin to go mainstream, we need to be able to pay from our cell phones. Modern (even near future) download speeds can't handle the blockchain size.
The logical end of the reduced block chain idea is a device that keeps none
of it locally, possibly not even the headers. There are dozens, maybe hundreds, of threads here with ideas on how to do that.
Oh, and I can move 8 gigs into my ancient Palm Treo in about 2 seconds using the SD protocol. Modern phones should be able to do even better.