Yes when the server is located within the united states and you have a warrant and federal subpeona it is quite easy - assuming the system keeps logs in the first place. As far as extracting logs from a router or switch, forget it - there is imply too much info generated too quickly so these are frequently discarded. The biggest point alot of you guys seem to misunderstand is that there is a tremendous (nearly impossible) burden in handing a subpeona over to an operator of a proxy/isp in nation which is not bound by extradition treaty to the united states. Basically, there is absolutely no incentive whatsoever for the admin to comply, it is just a waste of his time. This is why computer crackers typically hop around the globe through remote computers - the more jurisdictions their packets transverse, the more impossible it becomes to ever trace them back.
BayRidgeNY: You make some good points, but nearly every potential problem you mention can be addressed adequately by someone with only mediocre computer skills. First and foremost, employ an encyrpted proxy to hide the content of your packets from your isp. If the man is listening, most likely the carnivore box is at your isp. Secondly employ an encrypted webmail account from either hushmail or cyber-rights and never, never, never open any attachment sent to you by anybody.
If you are truly intrested in good computer hygiene, then ditch windows. Every lame attempt by the feds to implant key-logging worms on your computer is based on the assumption that the target is running some flavor of windows. Since PGP and similar clones have become so widespread, the feds are really desparate, since they cannot crack your public key, they have to attempt to recover your pass phrase by attempting to place a key logging trojan on your computer. Again if you simply ignore all email attachments you'd normally be safe. It seems now however, the feds now may legally break into your house and attempt to place software of your machine without you knowing (and with very little oversight from the judcial branch). If they see your computer is running a securely configured linux or bsd distribution, they're fucked.