Technically, yes, they could void the warranty. So long as you can run the RUU to install the stock 2.1 ROM, though, I cannot believe that they would ever be able to tell that the phone was rooted.
Also, I do not believe that anybody has ever reported that a rooted Eris with a hardware problem returned to Verizon was charged full price for returning a rooted phone. That said, technically, yes, you are doing something that invalidates your warranty. I have been running my phone rooted since late May/early June, and I have had absolutely no problems.
Thank You doogald. Whats is RUU and which ROM should I use ?