We've seen several motherboard designs with these empty spaces for a slot of some sort. It's no secret that this was to be the basis for Intel's Braidwood technology, essentially the evolutionary update to Turbo Memory functioning akin to an on-board SSD up to 16GB large.
Unfortunately, Braidwood and the pin-compatible big-brother to P55--P57--are no more. The good news for those who were planning to hold off on P55 in anticipation of P57/Braidwood is that you don't have to. The bad news is that we'll never know how well the technology might have performed. Initial reports suggest that it really didn't, though.