May be a power supply issue...hopefully.
Since it's being fixed under warranty, does it matter what the actual problem was/is? 🙂
Those new big quad-core iMacs chew a lot of power, so make sure you are plugging the power cord into a reliable power source. If there's a power strip, get a brand new one (or at least make sure it's not too old). If other devices like printers and external drives and power adapters are connected to the same power strip, get one just for the iMac (and maybe one or two low-power devices) and connect the other devices to another power strip.