@magicalclick: It's not quite rectangular, there's a slightly trapezoidal shape to the end of the plug (otherwise it would fit either way round!) I can't say I've ever known a usb cable to have a particularly distinguishing texture on either side. The Kindle one in front of me right now does have a slightly etched in USB logo on one side, I suppose, although I couldn't tell you off hand whether that's supposed to be on the top or the bottom - so I just end up comparing the shape of the plug and socket to ensure I've oriented them right.

And all of this is one of the reasons that, as silly as it sounds, I'm quite attracted to those Nokia wireless charging pillow things...