Microsoft won't let the HDD go. That's the tool they use to split the SKU's.
In fact, I'm willing to bet the slim X360 will have a hole in either the top or bottom plate to hold the HDD and it's case since the slim profile wouldn't allow for side-mounting as they do now.
I'd be glad if the slim had a SDHC slot they'd let us use as the "internal" X360 HDD but it won't happen, it would cost them millions in revenue.