I was planning on shooting an email over to the PlayBook support address this weekend, but if anyone has a better contact, be my guest.
I have had the freezing issue three times now, the first time while my files were already encrypted. The freezing presented itself shortly after I had moved several movies to a desktop as well as activated encryption, which led me to initially believe that the processor could not handle recording encrypted 1080p. That time, decrypting the files solved the issue.
More recently, the issue has come back while my files were already decrypted, but always after moving a large quantity of files to or from the PlayBook. Tweaking my previous solution, I tried encrypting the PlayBook, restarting, and then immediately decrypting it. This solves the freezing issue until, it seems, another large number of files are moved. It points to a possible corruption issue with the file system, but not knowing how the PlayBook's file system is arranged, I couldn't begin to guess what causes this. When I move recordings off daily (2-6 GB per day), it doesn't seem to happen - only when I fall behind and move over 10 GB at once.
You encrypt files through the Settings panel, under Security. It takes an hour or two for encryption, same with decryption. I set it to encrypt in the morning, reboot at lunch, then decrypt through the afternoon. The PlayBook can still be used while this is taking place, although I avoid file-heavy tasks like recording or downloading.
Edit: I wonder if this may also be behind the cause of some of the "slow WiFi" issues. I noticed that WiFi file transfers were down to under 200KB/s this week, and after the encrypt/decrypt I am now transferring at 600 KB/s over a G network. It is a finicky network so this may be a coincidence.