If you have the right kit facing out and know what you're doing you could work it out, but I don't. Since I tried not to let my IP slip out it would've been tricky to know who to target so it wasn't a problem for me.
On the other hand, I've watched someone do it to someone else, so I know
that was the real thing
I've also had someone hack my server because he couldn't work out my IP at the time (thanks to +x). Of course, we had logs and he bragged about it in public forum. The response from his ISP was... well, I don't know because they never replied.