Ok, this morning the PS3 greeted me with a "The correct hard drive cannot be found" boot screen, meaning the 320GB drive I installed previously was having an issue.
After a mild heart attack (despite my somewhat regular backups), and rebooting, the message went away. But for how long?
After some Googling, the solution lies on this page, in the second post from the top.
Basically I took the 'SATA1 jumper' off when I installed the drive, thinking it would perhaps interfere with any speed benefits. Turns out, older PS3s like mine need that jumper on so it knows what kind of drive it is (SATA1 vs SATA2). The PS3 uses a SATA1 controller, so that's what it likes to see connected. Newer PS3s may not exhibit this problem, since they have newer components which autodetect drives better, etc. There's more tech details in that link if you're curious.
Anyway, I've attached the jumper, and will see how it goes.