As with many rumors, they stand or fall with credibility. Credibility for a separate flash drive for OSX is very low, as this is not how OSX works. As mentioned before the home folder is on the same drive as the system, so an8 or even 16gb partition for OSX is simply not possible.
You're misinformed here. All of OS Xs folders are configurable.
The simplest way to do it would be symlinks. But some people on the "optibay alternative" thread have suggested that there is a better option - unfortunately I forgot how that worked and can't find the link right now.
Anyway OS X is easily configurable so the Users/ and maybe even /Apps folder is outside the SSD. There's also other system folders where stuff gets installed, like Library/Application Support. So if you go with 8GB you definitely need to move everything out that could fill up with normal usage.
Or they go the cache route, as you suggested. That would make things a lot easier and I imagine with some very simple smarts that would be quite effective. Example, the cache shouldn't cache any files that are bigger than a certain size - large files can load from the HD effectively. The cache should only hold small files that are accessed frequently. I can imagine if you did some tweaking you could arrive at a very good solution fairly quickly.
For a cache based solution I think they'd really want to put the logic in OS X - e.g OS X would know about the disk cache, and manage it. Then they can keep improving it / fix bugs with new OS versions. Maybe they just expand the memory cache to recognize 2 different caches, primary RAM cache, and secondary Flash cache. The Flash cache has the advantage that it's non-volatile so it can be a full write cache as well. RAM can't really do write caching very much because what if somebody pulls the plug.
Then, I'd add some logic by which system files I want to add always remain in the cache (are never removed)... and you get instant on etc
Whoop, sorry for the length. Designing a whole system in my head here - better wait for thursday, it's not that long out....