My dad leaves his old Inspiron on ALL the freaking time, and so far he hasn't seen any problems, save for about ten dead pixels that are his own fault. I forget what model it is, but it's about four years old. Just from that, I would trust dell in a heartbeat.
You should see how my manager treats the laptops at my workplace (RadioShack).. we have a Gateway, a Toshiba, and a Compaq all running in a locked plexiglass display case, running 24/7.. the Compaq has an Athlon XP-M and, due to the lack of ventilation, I think the BIOS was showing the CPU temp at around 60 degrees. Jesus, my XP 2200+ DESKTOP runs at 57 normally, and the XP-M is a low-voltage model. I put them all on standby one night after airing out the case, and he *yelled* at me the next day for it.. and still, all three units are running fine, *so* far. Even the LCDs don't have any dead pixels, and the Gateway and Compaq don't even have screensavers set up, so people can "see the desktop"... as if they haven't seen one before, or we couldn't demo it to show things like the LCD response time, which is more important than just staring at the screen. When you open the case, so much heat escapes that you instantly sweat.. I'm not joking, it's literally a sauna.
In short, if my dad can put his Dell, and if my neandertholic manager can put three laptops through two and a half months (and counting) of constant Hell, then I wouldn't have any worries about your laptop dying, say, two years down the road, so long as you don't block the vents, etc. Any problems will show up in your original warranty period, or, better yet, immediately.
In fact, if Dell dropped the automatic three-year warranty from the M60, I'm sure a lot more people would flock to it. It would be nice to see Dell offer mobile workstations to regular consumers without the next-business-day ewp forced down your throat. Yes,m the 8600 is basically a workstation, but maybe they can redesign the chassis? I happen to think the new style is a little new-age plasticy.. childish. Apple got it perfect, why can't anyone else?