QUOTE(The_Oldie @ Sep 28 2007, 07:58 PM)

Your MAC address would probably still be logged by the ISP, so if it was a police matter, they could probably still trace you.
If you were using someone else's wireless LAN you'd more than likely be using a soft/spoofed MAC address... so no worries there. And it wouldn't be logged by the ISP. It *might* be logged by the router but all an ISP sees is the router - not the individual machines on the LAN/WLAN.
Anyway, I think we're a little bit off-topic. The real answer is YES...
If someone who worked for Northern Rock posted inside information on this site and it was of significant interest for the FSA to engage the police to look into it then YES, the police could demand the admins of HPC.co.uk release the server logs to show the IP address(es) used to post comments. They could then be traced back to a (normal) user's broadband account or a company network. Wander round to that user's house and you'll probably find a cookie or two on the user's computer that would confirm this.
Simple rule - don't post anything you wouldn't be comfortable stating in a "real life" public forum.
Edited to say: Of course, these activities can only take place if something has occurred which is deemed potentially illegal. Northern Rock, for instance, would have no leg to stand on in demanding HPC.co.uk admins release data. Only law enforcement agencies (ie the police) can do that. So if someone had said "Northern Rock is fecked" they wouldn't have committed a crime, so would be fine.
However, if they were, perhaps, stupid/naive enough to use a Northern Rock corporate PC to post the comments then it would be a trivial matter for Northern Rock's IT folks to look in various local and/or proxy logs to track them down.