A few years back, when routers still cost a bunch, my first SMC Barricade wireless router with print server was as solid as a rock. My pc downloads 24/7 and apparently that takes a toll on routers. It lasted almost exactly a year before it died. Repaced under warranty, that unit lasted almost exactly a year before it died. I replaced it with a rock solid Netgear wireless FM114P with print server, solid as a rock also, meaning lots of heavy metal. That unit lasted almost exactly a year before it died and was replaced under warranty. By this time rebates and deals and 802.11G routers becoming common so I picked up an inexpensive D-Link to keep as a backup. When the Netgear died, I didn't bother with a warranty replacement, just like I gave up on the SMC finally, and pulled out the light-as-a-feather wireless D-Link router. Yep, almost 1-year later, it gave up the ghost.
Each of these units has always been connected to it's own UPS system and are in an air conditioned room, so I doubt it is heat, spikes, brownouts or lightning. I just think that they aren't built to last if you use them 24/7, and it doesn't matter if they are light plastic or heavy metal.
Picking up a Linksys WRT54G with rebate today, then I will look for the next rebate deal to keep a backup around!