Here are my thoughts on something I think might be interesting.
What if operating systems are installed on your 8 gig SD card? Yes, any fully installed operating system like Windows 7, Snow Leopard, Linux can be stored in an 8 gig SD card. The prices for an decent 8 gig SD card costs about $30 or less.
And what if these SD cards are locked and made not writeable except for system updates? Any changes made to the operating system will be on an normal hard drive as per normal. This prevents any malicious code modifying the operating system, and it also keeps the operating system clean and pristine. In fact, returning back to the original system will only require a clean wipe of your hard drive OS settings. But any security or system updates are still kept, so you don’t have to reinstall all those updates and drivers again.
Sadly, the read speed of the SDHC and SDXC aren’t high (but I think higher than normal hdd?) enough to make this possible and realistic enough. Maybe in the future. Who knows if this idea will become the norm.