Without knowing what a sound guerilla is, really, I can suggest that there's a microphone attachment for the ipod and there are lots of tiny flash memory mp3 players with built-in mics. Most pdas have integrated recording, too -- grab a an adequately large memory card and you're set.
But those are all just for lectures. If you want a good-sounding recording instead of a merely intelligible one you'll need something like this and a decent mic. There are cheaper, bulkier but basically similar units out there, too, this is just the one I remember seeing most recently.
In fact, most mp3 players with a decent feature set (ie, not the iPod) have built-in voice recorders.
Someone who does a lot of interviews for research recommended the old i-river iHP (120 or 140) for their digital recording quality. My wife has used one for lots of long personal interviews, and it works well. I don't know how it would work for public lectures. I assume the current i-rivers continue the tradition of high quality recording.