I find it odd when people make blanket claims about the lifespans of current-generation Intel Macs being shorter--the oldest Intel iMac in existence is only 4.5 years old now, so it's not like you'll even be able to offer a direct comparison for a few years yet. Now, the G5 iMacs had widespread problems with their power supplies due mostly to industrywide capacitor issues, but I haven't personally seen any indication at all that the Intel models with the same form factor are anything but more reliable.
Certainly, comparing the older Intel iMacs with the G5 iMacs we have installed where I work, the Intels have gone longer than the G5s without issues (both of our G5 iMacs had their power supplies fail with the capacitor issue after 3.5 years of use, repaired out of warranty for free and now fine); our two original-gen Intel iMacs were fine for 4 years, at which point one started to develop monitor lines. We have four ~3-year-old AL iMacs, none of which have, thus far, had any issues at all.
Not saying that it's not possible they're going to be less reliable in the long run--if nothing else, due to drastically higher thermal load I'd expect them to not fare as well as the VERY cool-running G3 iMacs--but it's rather early to say. And if anything, based on my personal experience, I've seen the quality control go UP in recent years, not down.