That's 90C and very close to the top temperature of the CPU. But if the heatsink is picking up that much temp, the CPU or whatever component is connected to the heatsink is even more higher and that's definitely not good..
There is no "breaking in" period with computers like cars. These are not mechanical things that need to have groves and so on..so please don't confuse the issue..
How are you checking the system temps? Grab the free
iStat nano, a Dashboard Widget by iSlayer and install it. Now open up the Dashboard and enable the iStat Nano widget if it isn't visible and take a peek at the temps.
On my 27" i7 iMac, I get the following numbers:
HD: 36C
CPU: 32C
Ambient: 14C (crazy cold basement)
GPU Diode: 46C
GPU Heatsink: 43C
Mem Controller: 35C
Optical Drive: 30C
Power Supply 2: 45C
Under crazy load, I've had the CPU go to about 60-65C..but that's about it..
Regards