Amazon S3 – “Simple Storage Service” – is coming very shortly. Others will no doubt follow – talk about Google Drive has been floating around for a bit.
The trick with all of these services should be that many people back up a lot of the same stuff. The same DLLs, the same MP3 files, the same application binaries. An intelligent client should be able to checksum the stuff on your local disk and only upload it if it’s not already stored on the service by somebody else, in which case it should just upload a pointer. Likewise, many files are very similar, and some intelligent differencing should mean that only patches need to be uploaded.
Your PC will soon just be a cache of your online world…