It looks like the "shutdown bug" that has plagued my iMac in the past has returned. I suspect we won't be as lucky as we have been in getting it repaired for free, so the only way to fix it for good would be to get it replaced. Quite frankly, after almost 5 years, it's about time to get an upgrade anyway.
We looked into the available iMacs (which are pretty sweet, by the way), but we just can't afford one right at this moment. Once our 2008 tax refund gets here, we'll be in better position to get one (with Donna's U of H student discount!), but right now, it's not possible.
I have a theory, though, that keeping the iMac cool will help reduce the sudden shutdown problem. So this weekend, I experimented with keeping an ice pack just under the air intake of the iMac. It did indeed slow the fan down (the speed varies depending on the internal temperature), but it's hard to say if that prevented a sudden shutdown. So far, it's happened twice since Friday, but not at all since I started using the ice-cooling method.