I owned bitcoins at mtgox during bankruptcy, am I a creditor and thus have to file a proof of claim?
From the mtgox website:
Please be aware that confirming the balance on this site does not constitute a filing of bankruptcy claims under the bankruptcy procedure and note that the balance amounts shown on this site should also not be considered an acknowledgment by the bankruptcy trustee of the amount of any bankruptcy claims of users. Bankruptcy claims under a bankruptcy procedure will be fixed through filing of claims and the investigation procedure. The method for filing claims will be published on this site as soon as we are in a situation to announce it.
This implies that yes, you as a creditor will have to file a claim to avoid forfeiture, even if the bankruptcy trustee already knows about it from mtgox' data, and even if the current website confirms that to you. There is a deadline for filing your claims that apparently has recently been moved to May 29th 2015. The last sentence from the quote suggests that the trustee is struggling to provide a means of filing claims that scales to the thousands or tens of thousands of creditors.
All of this is based on taking the information at the mtgox website at face value. A better answer would include whatever big money creditors or their lawyers say---but I have no source for that.