I'm doing this already, it works bloody well. I have my OS and Programs installed on a 120GB SSD, which sits around 50% utilisation and use NTFS junctions (aka symlinks) to map storage for stuff that doesn't need superfast seek speed (aka data) onto a group of 1.5TB drives. It takes a little management, so isn't quite ready for the average user yet - but you do effectively get something like 5TB of online disk space combined with SSD performance.
Interestingly, i've found on Windows 7 that by running OS/Programs from the SSD that the contention issues you would normally get on a spinning disk are mitigated a great deal - and there's no noticeable hit with having the entire user profile (including junk like web cache). The system is booted in 5 seconds after finishing its POST, and the desktop is snappy right from the get go.
As to the hard disks as backup, it works pretty good. At work we have maybe 1.2TB in a full backup - we do a weekly full backup and incrementals onto LTO tapes, and a second weekly backup onto a consumer grade SATA 1.5TB drive in a USB cradle. The SATA drives are taken offsite in case the office burns down, £80 + carrying a few hundred grams around makes for really cheap and fast data transfer.