I do a lot of voice recording, and although I find the quality limited, the Creative Muvo TX (and it's near-equivalents) works very well for me as an ad hoc USB key, MP3 player and voice recorder. As a pocket-sized recorder which you can plug into any USB socket (and drag the recording off to a PC), it is hard to beat for convenience. These days I keep it in my bag, mostly empty, so that I can record whenever the need hits.
The iTalk is the easiest to use recording solution for a mainstream MP3 player on the market, and it's acceptable (on the lines of the Muvo TX) quality for voice but hardly high fidelity. When I had the 3G iPod (I still have the iTalk, perhaps awaiting a future re-iPod) I found it a very versatile and productive solution (sync voice and if necessary burn to CD within a few mouse clicks) ... as long as the battery held up. With the 4G, I expect that to be less of an issue, and the Photo I suppose makes that pretty much a non-issue for a couple of hours of recording.
The other MP3 players are an option, especially the full-size iRiver and iAudio players. These feature decent recording capability combined with a usable Line Out, which you can listen to in the car. I found the iAudio M3 marginally more usable in the voice recording mode, and it does a good job.
I have a Hi-MD, and although it is IMO a very good device to use when you want things to be in better quality, honestly I never use it for voice recording wher immediacy is more important to me. Having to remember to take blank disks, choosing a stereo mic which is compact, remembering to take the Mic, etc is a pain, and that's before we get to Hi-MD's limitations as a mass storage player.