I went out shopping for a basic model, and settled on what turned out to be the new Shuffle model. And it is bloody tiny. About the size and shape of a moderately showy tie-clip, it'd be worryingly easy to just lose it, or even blow it away with a good sneeze. If you really wanted, you could hide it in various places around your person without too much awkwardness, or even have it fitted subcutaneously. Blimey, it's small.
It holds 1000-odd songs (albeit as just-about-adequate MP3s) and has a claimed battery life of 10 hours. The memory isn't all that impressive - it's just a flash chip, and I've got equally diddy camera memory cards and USB drives that hold more than this gizmo's 3.8GB. It's the battery I'm more impressed by - I've not used it to its limits yet, but it had no problem with seven-odd hours on the trains yesterday. It's apparently a 3.7V lithium-polymer battery, in which the electrolyte is held in a solid polymer.
Here's how it all fits together, courtesy of the techno-vivisectionists at iFixit.
There's some damn smart engineering gone into this. And it cost less than the day's train tickets.