Perhaps it was misworded on your part but a wifi hotspot doesn't actually "hide your IP you get from your ISP" since you're using someone elses ISP your own isn't even involved and therefore isn't in the picture at all.
"...but even there traces may be left. The hotspot, (internet cafe, local hotspot, whatever), will still have transactions showing your access and with a little work such can be traced back to specific machines." Not that the person who started this thread was interested in doing something illegal, but for those people who do such things someone can, for example, make a bomb threat to the White House and a direct threat to the President using a free wifi connection somewhere and not get caught. At most the wifi place 'may' have a video camera installed and manage to capture him/her on cam - at best they'd end up with the person's description, unless the perpetrator can be followed home immediately after the threat is made the person would never get caught - never, unless the person went back to the same place and used the wifi again and a sting had been set up ahead of time.
As far as traces being left on the perpetrators machine in the above example, computer forensics would find any, if there's any to be found assuming he/she didn't take care of that in the meantime or beforehand, after-the-fact if the person had actually went to the same wifi spot and got caught in the sting that was set up. Of course, he/she would've already been caught by then and anything found with forensics would just be used to help seal the case.
Of course, what I just said in the paragraph above brings computer forensics into the topic (not surprisingly) and that's a whole other ball game - since it is, I'll not go thoroughly into that topic lest this thread not end any time soon... haha.
Interesting it is though. Very interesting.