I've seen several machines checked in over the last few months complaining about fast fans, that didn't experience problems here. I remember several of them having carbonite installed. Is there a known issue there?

Or can I safely assume that it will hammer the CPU (and spin up the fans) when a backup runs? customers are complaining about fast fans.

Possibly. It seems that Carbonite (read: CarboniteDaemon) sucks up CPU cycles at a high rate when scanning for files, particularly running apps. Fast fans are associated with this kind of CPU usage. Lion is also frequently mentioned in this context. Some users suggest to exclude the Applications folder from backup scans (via the Backup tab in the Carbonite prefs panel) to avoid having Carbonite scan running apps.

I realize that mac users are accustomed to their computers being quiet when not being taxed, unlike say, a dell desktop that just drones 24/7. so for them, the computer suddenly spinning up for awhile could be cause for alarm. I suspect that's why I'm seeing these.

I was just wondering if this is a more widely-experienced issue? When a computer is checked in for fast fans and it's not acting up when I see it, it can be tricky to figure out / speculate / guess wildly what's causing it.

I'm just seeing carbonite on almost all the fast fans I've seen lately so I assume it's somewhat widespread?

I couldn't say if it's widespread or if so how widespread. All you can find is users complaining about fast fans, often under Lion. Some claim that excluding apps from backup scan helps cutting back fan activity. That's about the extent of things, sorry.