What is that exactly? A standard 3.5" drive or one of the smaller ones? The smaller it is/smaller the case, probably the more of a chance it'd overheat. I know my 3.5" enclosure gets pretty hot after a while, and it's even got a fan!
Similar to this? My mentor had one of those and it never got hot. A case like that is almost like sitting the disk on your desk But most others don't get any air flow. If that's what you have, I don't think it would have any heat problems. I still wouldn't run it ALL the time just to help keep the disk longer. 2.5" disks die rather quickly when compared to 3.5"
a 40Gb disk you say... then i think its a 5400rpm one,.. dont worry, they dont really get hot. i mean... i saw people putting their 15k scsi disk's into tupperware boxes, for noise reduction, and they still worked fine without overheating. so dont worry about it