Hi all. New to the forum, but have been lurking for a while.
Couple of things.

1) Any ISP (Including Earthlink, AOL, Erol's, Joe Bob's ISP, etc...) (or anyplace for that matter[stores, credit card companies, banks, etc..]) MUST release any and all info IF presented with a warrant. Without a warrant, it is up to the individual service and thier polies wether they release the info freely or not. (Most won't without a warrant).

2) A dynamic IP DOES NOT protect you!!! Most ISP's keep log files of ALL activity. With dynamic IP's the log looks like something like this:

Incoming call on modem TC12
Answer call
Authenticate call via username and password.
Allow access and log phone number (If caller ID is available and setup)
Assign IP Address 206.234.24.56 to modem TC12.
Log date and time
End call (When user disconnects)
Log date and time

Now you COULD argue that it wasn't you calling, but if caller ID is available it would be hard to argue.

Now lets assume caller ID isn't available, but you did call from your house. Could they still track you? YEP! Here's how:
Using the log above as a starting point, they check the username and look up the account holders info. They then call the phone company and get a list of all phone numbers associated with that account holder. They then have the phone company do a search for the dates and times in the log, to see if any of those numbers called the ISP at the same time. If so, then they got you. If not, you get away for now.

The above assumes a few things:
1) They have warrants for everything.
2) They even want to bother with this.

So while they COULD do it, for the most part (at least 95%) they won't bother since the end result (1 arrested user) isn't worth the time/energy spent.
If that user is part of a warez group then sure they might do it, but otherwise no.

And just to reiterate, with a warrant ANY company has to give up the info, otherwise they can be hit with obstruction of justice, interfering with a federal investigation (Which warez falls under, it is technically a federal offense) and more. If an ISP really pissed the Feds of by not cooperating after a warrant is served, they could probably get an injunction to shut them down.