I cant understand how people are so baffled about the DP and warranty. You know why its like that? DP might have some bugs and the service centers are not owned by MS, so when people take their handsets for warranty, someone else pays than MS. As DP has not been tested by Nokia,ATT,Verizon whoever, you cant expect them to fix those handsets for free.
That said, you can use a flash tool to revert back to older rom. I have reverted back once with my Ativ S. Nokia has NSU (nokia software updater) for this process. If you want to be sure, before the upgrade download the older rom and NSU so you have them ready if you would need them.