After trying it for three weeks without problems, I bought the latest version of PGP Desktop Professional, which includes whole disk encryption. Both my MacBook Pro's internal hard drive and the external drive I use for Time Machine backups have gotten along with it just fine, even through the system update to 10.5.7. For the most part there's no noticeable impact on performance, but then my laptop doesn't do anything really disk intensive--all my photography work happens on a different computer which I will not be encrypting. There did seem to be a slowdown in Time Machine backups, but that's not an area where performance is really relevant. I would really prefer to use TrueCrypt, but as it currently can only do whole disk encryption on Windows (where I have been using it for some time), that wasn't an option.
The rest of the PGP Desktop package gets a mixed review. I had looked at PGP last summer and dismissed it as unacceptable because of the horribly designed proxy it relies on for encrypting email, but this time around I discovered that there is also an officially unsupported plugin available for Mail. The plugin works the same way as the GPGMail plugin, but with fewer features. This is not surprising, as they have the same author. Apparently some brainless product manager at PGP Corporation had decided to kill the plugin (presumably to force users into using their worthless proxy), and it was brought back by popular demand.
Since the last time I looked at PGP, it's lost the ability to communicate with public key servers other than the one actually run by PGP Corporation, which very few people use. According to a thread on the PGP support forum, the developers know about this bug and just don't care about fixing it. Well, nobody will ever accuse the PGP Corporation of having good customer service or QA! Fortunately the keyservers have web interfaces so the problem can be worked around as long as you're using the "unsupported" Mail plugin. Anyone foolish enough to use the proxy will be out of luck, though.
I ultimately decided to switch from GPG to PGP for my email needs, at least for the moment, because while both of them have huge problems on the Mac, PGP's refusal to work with keyservers that aren't owned by the PGP Corporation is less of a problem than the hideously unusable keychain management that GPG inflicts.