Without mentioning any names I will tell you that I have read in many forums the following declaration: "Every Apple Time Capsule" running 24/7, purchased in 2008 will be dead by 2010".
But, I can tell you that these reports of death have been greatly exaggerated... I recently installed a Seagate ST4000VN000 4TB HDD into my FIRST GEN Time Capsule (Model A1254), and everything has been working perfectly as before. Not only that, but this is my second drive upgrade for a venerable piece of equipment.
I bought this Time Capsule in late 2008 and used it with the original (Hitachi 1TB) drive - 24/7/365 in a poorly ventilated NYC apartment with no climate control – for about 2 years until that drive became too small for the backup needs of 3 machines. I installed a WD20EARS 2TB in late 2010 using the excellent tutorial described here at iFixit and that drive worked just fine until it conked out about 2 months ago and began reporting backup failures. Without a second thought I cracked open the TC again, installed the new Seagate NAS, and began backing up again in about 10 minutes.
I do subscribe to the notion that too many backups are almost enough... so I have redundancy options in place to avoid data loss at all costs and I have no doubt that TCs can/will fail prematurely from overheating (as an Apple customer since 1998 I have absolutely had my share of failed devices for any number of reasons). Don't leave yourself in a position to learn the hard way, however, there is no reason to avoid this easy upgrade if you are replacing with a comparable drive. My only concern was whether a drive higher than 3TB could be recognized by the hardware. So hopefully this helps out someone else who has the same question as me.
As a side note: The original 1TB Hitachi I pulled out of the TC has outlived the WD drive I replaced it with! I am currently using that drive for media storage in a Newer Technology Ministack v2 attached to my 2007 Mac Mini (functioning as home theater PC).