Of the few of those that have "one button recording" at all, they generally still have slide-and-hold or otherwise complicated mechanisms to power on the device before the one-button part works, or they have a slide-and-hold record switch that doesn't give good feedback that they're actually recording. (The VP-20 is like that.) Hmm, the voicetracer looks plausible, you just have to push the switch one way. (Most of the pen-form ones have the secondary flaw that they're trying to be stealthy, which is illegal in ⅔ of US states.)

The one consumer recorder that I've found that's really "grab it, press one button, start talking, and know you're recording" is the teenage engineering TP-7 - which is an amazing bit of engineering, but for $1500 it had better be.