If I understand what your talking about here, a SSD that plugs into a empty RAM slot.
I am not sure if a RAM slot connector can be used for a storage medium like a SSD is used for. SSD's are not being polled every ms like RAM sticks are, but rather only when needed by a storage request.
The speed of chips used in SSD's is also much slower than RAM sticks so it could be a huge bottle nexk in memory speed's. Picture a SSD with 2,000mhz rated speed and being able to tweak it in the BIOS, yummy
Right now tho that PCIe slot is faster than what a SSD can handle. But I am not sure how SATA3 will effect SSD speeds at this time. I don't know if there is a SATA3 SSD out there.
A better solution might be one freaking huge DIMM on a 64bit setup that off loads the whole OS into a RAM drive and then uses it. In fact the whole Win7 OS on a chip with be super cool and extremely fast. No more DVD's and installs, just plus a new chip in and away you go.
I remember back in the Amiga days I had a nice RAM expansion card that on boot up I transfer several compressed floppys into a Recoverable RAM drive and made all assignments so the OS was run from there. It was a huge improvement over even a HD setup.