I mentioned last week that I had started using PGP Whole Disk Encryption on my laptop (a first generation MacBook Pro 17" btw, it has a 32bit 2.16Ghz Core Duo with 2Gb RAM).
I encrypted the external FW800 500Gb hard disk that I use exclusively for Time Machine last night. It took about 8 hours. So far so good.
When I connect it to my mac, a PGP WDE dialog pop-up shows up, asking me for the correct pass-phrase. After I enter it, it shows up as a Time Machine external disk, and the backup starts.
Be aware that, in case of hard disk disasters, encrypting your TM disk can be more awkward than encrypting your internal hard drive.
With a normal non-encripted TM drive, you can boot your Mac from a Leopard DVD, and ask the Installer to restore a TM backup directly.
If you encrypt you TM disk, thats no longer an option. You have to install a bare bones system, install PGP WDE, and then restore the TM backup.
A possible solution (untested for now) is to have a clone of your internal hard drive (even an encrypted clone should work) that you update from time to time. Then you should be able to boot from the clone and restore a TM backup to the internal disk.
As I said, I haven't tried this yet. It should work, I don't see any reason not to.
I do have a clone of my internal hard drive, thats the next one to encrypt. I'll check to see if he is still bootable afterwards, and I'll try to find an extra disk somewhere to restore a TM backup.
I'm really happy with PGP WDE, works great, no surprises so far.