Can you be more specific about where on the phone the red X actually is?
On the screen of the phone? On the battery icon? Or physically on the phone and /or battery? Did you let the phone completely discharge (go dead)?
My first thought, without knowing more specifics is that the battery is completely dead. If this is indeed the case, you need to let the phone charge on the home charger (don't even think about trying this with a USB port on a computer) for 8-10 hours at least.
It could also mean that the battery has gone bad, can't be charged, and must be replaced with a new one.
My best guesses at the moment.
I have 2 droids, and they both have a red x near the battery location. They were there when I got them last year, both brand new. I have not had any problems, but it should not void the warranty if they came that way. I checked with my Verizon store about it, and they said it take s no moisture at all to turn them red, so it is determined by the problems of the phone if they were caused by moisture. Just thought I would share.