Nurta,
Couple things are likely going on at the MPAA now that you have received that notice;
MPAA is likely moving for a subpeona to get your contact information from Comcast
MPAA is likely creating a settlement offer for an outrageous amount of money
You should read up on the methods/processes they used to identify you as the offender. Because they tend to use crappy methods, you can ask the court for summary judgement that you did not commit the offense since they cannot identify, with specificity, that it was you.
Then, move for discovery using the questions here in the linked PDF
Pike & Fischer Internet & Law Regulation
This PDF deals with the RIAA but they use the same methods to identify people so it applies here. Also read up on the Oregon Atty General's investigation into the RIAA based on, what he feels are several offenses they committed by improperly investigating the person referenced in that case. Your state may have similar laws preventing the MPAA from utilizing the methods they used. (i.e. practicing private investigation without a license)
A very good source of information can be found at
Recording Industry vs. The People
The easiest way to avoid this kind of thing is not to upload movies ;)
If they cannot identify, with certainty, that it was you, move to dismiss with prejudice.
Of course, the easiest way to deal with this is not to upload a crappy movie in the first place ;)