I think it's true. I heard it on the news a day or so after it happened, on Los Angeles stations KFI and KABC. They did say that it happened in Tustin, I think at a mailbox drop place (like Mailboxes, Etc. - they did not give a name). A man came in with many envelopes addressed to hospitals and wanted them stamped for postage, and they were cold to the touch, as if they'd been refrigerated (weather was quite warm, too), with no return addresses. When the person behind the counter asked him to put return addresses on, he refused, grabbed the letters, and left.
However, no one has ever said anything about these letters again. I wonder if they got mailed.