Depends on the product. At the high end, like the Mac Pro, internal memory, processor, and storage can be upgraded. I haven't seen any video upgrades yet, but the video card isn't soldered in place. MacBook Pro SSDs and batteries can be replaced, if not easily by the user, same for MacBook Airs. (Yeah, the battery is glued in place; that's what heat guns are for.) iPod, iPhone and iPad batteries can be replaced, if not trivially easily.