The hard part is not downloading or storing the block chain. It's building, maintaining and guaranteeing the on-disk consistency of the block and transaction index that is necessary for validation.
Well, I suppose you've given it a lot of thought already. But I still have a hard time believing downloading the block chain and creating an index requires all that disk hammering. On my computer, the .bitcoin directory is 1.6GB big. It's big, but not that big. It even fits in the memory of most computers! How can building it require hammering the disk for hours?
It takes about half an hour for me, when loading from a local file (a patch that will probably be in 0.7) instead of from the network.
If you have a lot of RAM, you can run it in a RAM drive (if your OS supports that) and watch it fly. Of course, you'll miss the one hard part: not losing or corrupting the on-disk index when your computer crashes.