I'm not exactly sure, to be honest. It's possible that you can void your warranty, but I'd say that the benefits of rooting + custom ROM far outweigh the risk
The risk of bricking is quite small, because the phones are pretty much identical and the developers test their solutions a lot before releasing them into the wild. Still, it's a good idea to read through forum threads to see what kind of issues people report, and ALWAYS read the warnings and instructions before you touch anything.
Yep, I understood why you were interested in unrooting, and I was saying that it probably won't be necessary to do so. Then again, it might be safer just in case they try to blame custom software for hardware faults.