I had a kernel panic about 2 years ago, and another one a few days ago. I haven't had any other problems besides minor software bugs, and I was able to harmlessly reboot after both. I've been told this can mean many bad hardware-related things. Could it just be something relatively harmless, since it doesn't really seem too persistent? Is it worth having it checked out?
The first time you get a kernel panic, it is a reminder to make sure you have a regular backup system in place.
Other than that, until you lose real work or have quite a few, there isn't a real concern other than annoyance.
When you notice a pattern or want practice troubleshooting, go for isolating why, but most users should just report the panic to Apple and reboot.
Yes, sometimes a kernel panic can indicate bad/damaged or incompatible hardware. The frequency with which you have them is probably not indicative of a major hardware problem (i.e. bad cable connected to MotherBoard from HDD, etc.), unless you notice physical conditions that seem to coincide (i.e. hottest day of the year, child pounding on desk, etc.), or while performing the same software related task (i.e. clearing caches). It could be as simple as having one system file corrupted that is not often used.
On the other hand, I have also seen this caused by having hardware just lose its connection slightly (i.e. an eSata ExpressCard slipping out just a little while in use on a 17-inch MacBook Pro).