I was just looking at ebay for sleeves for the UMB, and it seems that its either neoprene or memory foam. I think I like the neoprene better as it should provide a little more shock resistance than memory foam (if already compressed , eg. inside a bookbag). The problem with the neoprene is that it really doesn't allow heat to dissipate. My fear is, as with pretty much every PC I've had, sometimes after closing the lid, and freezes and keeps running, and heating up a storm inside the bag. The neoprene will not allow for any of that heat to escape, thus cooking the macbook. Is this not a valid concern for macs? What sleeve do you use?