I've had my run-in with this before. I'm just a generic every day sysadmin and have no real involvement with the security community, short of idling on IRC with a bunch of more active people. Here are my experiences:
In 1997 or '98 I was the sysadmin for a mom 'n pop local ISP. We got hit by a massive DOS attack - keep in mind this was in the pre-smurf/DDOS era, so it really did warrant the attention of the feds. The owner contacted them, and they talked to me about getting any logs we might have (which of course I was ready to provide). I asked them where they wanted me to send them, and... "No, why don't you meet us out somewhere? We'll buy you lunch.". Despite the offer of free food, the alarm bells were going off by this point. So, I met them at a local coffee shop, and out of the 30 or so minutes I was there, they spent maybe two minutes discussing the DDOS with me, and the rest of the time attempting to get me to inform on the local 2600 group. I declined repeatedly, and they continued to make more forceful and threatening requests. Every time I disagreed with them, they looked at each other - and this was the creepiest (and obviously rehearsed) behavior I've ever seen. They never did get those logs from me.
After that I didn't hear anything until around 2005 or so when one of my ex-coworkers from another company called to tell me two men came by looking for me, and that they had government plates on their car. They left a card, but since I'm not under any obligation to call them, I never did. As the years went by, I received more calls from different people with a similar story.
And my last run-in with them was only a year or two ago - someone called me from a cell phone claiming he was with the FBI, and he had my computer and I needed to come to the local field office to pick it up. I found that to be rather unlikely since I tend to hang onto them until they're dead, I certainly wasn't missing one, and then they (minus the drives - I still have those) go into the bin. After a week of ignoring his calls he stopped bothering me.
To this day I have no idea what they wanted, but the entire thing reeked of ill-spent tax dollars.
I really don't care anymore, so the hell with posting as AC...