What is sad is that, as others have said returning the CPU to the factory clock-speed is very UNLIKELY to damage the cpu. BUT simply having it overclocked or rooted will void the warranty.
Being that I was the first that I know of to release a kernel with overdrive enabled to the public, let me assure you that it is safe and mostly harmless.
I say mostly harmless because it does harm battery life a bit if you are running at full tilt all the time, and there is always a very miniscule danger of something going wrong in any system.
The TI hardware specifications for the omap arm processor in the droid list 600MHz as an acceptable safe frequency, so it is at least aproved by the company that made the processor itself.
Motorola most likely ran at 550MHz because they wanted to increase battery life, and increase stabillity by a hair. It is a known fact that the slower a processor is ran, the less power it takes, and the more stable it is.
I for one have not noticed any stability issues at 600MHz that werent also present when running stock, so before you blame anything on the overdrive mode, throttle back to 550MHz and see if it is still there. Chances are it will still be there, and be unrelated.
As to verizon finding out if you root your phone, if they can find out you rooted the fact that you have a custom kernel or not is null anyways.
One small clarification, overclocking on the droid is soly relient on the kernel, mainly the cpufreq driver. There is no bios on the phone, except maybe the bootloader/spl/radio, and those are not touched. Not nitpicking or anything, just sharing information.
Actually heat is more dommageable to entire phone, as the omap3430 can bear 90 degres C when at 600MHz. Some samples have reached 900MHz in lab conditions... I don't encourage you to reproduce, but 600 is a frequency guaranted by TI. Often most of the part of the phones doesn't support this temperature (it doesn't help to cool the battery and circuitry doesn't support 90 degress for sure.
Well of coarse Rooting voids your warranty, but there are several methods to return your phone to a completely stock device. Returning the Droid is not a problem if you Root.
Is it likely that we will ever see a kernel that allows the cpu to run at something higher than 600 mhz? I'm not asking if it would be good for the phone or not, just wondering it's it's possible and likely to happen eventually.