All good points...couple additional thoughts for the OP.
Say you have a problem with your phone that DOES constitute a "valid warranty" return. You don't want to send a rooted phone back to Verizon. As long as the phone is functional (say for example, the spacebar no longer works on your keyboard)...You can flash back to complete stock (no root, no superuser app in the app drawer, etc.). This way, they would NEVER be able to tell that you were rooted and your valid warranty would stand. And you're not cheating the system because being rooted has nothing to do with your spacebar failing...so you can sleep at night with a clear conscious
Now if you burn your processor up for example (which I have yet to hear about), you could still try and cheat the system by telling Verizon..."It just won't turn on" and they would probably send you a warranty replacement. The problem with that is Motorola might find that the problem is a burnt out processor, replace it, and then see all the "rooted goodness". You would then get charged full retail price for your Droid.
My advice, if you are Rooted and YOU cause the problem with the phone...throw it in the pool, or run it over with the car, pay the $50 or $75 insurance deductible and move on. If you try and cheat Verizon and they catch you...it will cost you much more.