Step 1: Get your stuff: Nomad
With a little shopping around, however, one can put together a unit that meets requirements. This is possible for one reason, and one reason only: the Creative Nomad Jukebox 3. This product, long discontinued, is the result of audio engineers known mostly for their sound cards, interfaces and other geek toys being turned loose on the consumer mp3 player market. The resulting device is the antithesis of the iPod: heavy, built to resemble a portable CD player, and covered with ports: a headphone port, two line outs, and a line in that accepts optical and 1/8", FireWire, USB, 5vDC and IR port (turn this off). It is that optical port that enables the one function which makes this the Must Have Item: the njb3 records optical 16 bit signals as .WAV in 48 and 44.1 kHz stereo. Hallelujah.
The njb3 has other key advantages: it comes with one lithium ion battery pack, but can be converted over to two, giving an actual six hours of record time. Equally cool, it has a standard 2 1/2" laptop style hard drive and can be easily upgraded. The stock 20 gb is good for 30 hours or so of .WAV, and it's all capacity from there.
The optical port is the main thing, though. Many mp3 players, including the njb3, can record line-in sound or even provide a preamp. The resulting recordings suck: the A/D converter is the cheapest one they can find, and rightly so, since the hard drive noise is going to ruin your recording anyway, as will the electrical noise. This is not the way; although this rig will allow you to run in a 'stripped down' mode that looks a whole lot like listening to a CD player, the quality must be improved by an outboard AD converter in order to meet our spec.
The njb3 is no longer produced, but many new-in-box units exist and creative still sells batteries off and on...someone will eventually pick up an aftermarket for these batteries if Creative drops the ball, because as the eBay price for a used one will show you, these puppies are coveted. As for other mp3 players that can do the trick, Neuros is waffling on a digital in for the Neuros 3. Express your preference that they do this, because then we will have no longer to deal with the quirks of a long out-of-production proprietary codebase. There are no other contenders, to my knowledge. Your Nomad should cost around U$300.