I have 3 external FW drives attached that I leave on all the time. 1 of them is for my Music, and the other two are for nightly backups that I run on my PB. I see no issues, and have been doing this for over 2 years. Of course things will fail eventually, but nothing lasts forever.
I have noticed, though, that sometimes OSX will not be able to access one or two of the external drives forcing me to disconnect them in order for OSX to run properly (otherwise I have to wait for OSX to time out while it tries to communicate with the device). I think this has too do more with what kind of chipset your external device has, as some chipsets might not play 100% nice with your Mac. I have also noticed that if I tell OSX to put disks too sleep when not in use causes more of these disconnect issues than if I were to leave them on 100% of the time. Sure it means I use a little bit more energy, but causes a lot less headaches.
Kimo