xda-developers forum has a post with the exact problem (frozen screen and question mark in battery icon). According to the post, Lumia 800 and Samsung Focus had the same problem until a new software was released to fix it. So hopefully one for the Lumia 900 will be released soon. WPCentral has several posts with other Lumia 900 screen issues, some of which Nokia has acknowledged and promised a software fix soon (no exact date). Nokia.com support forum also list several options to fix a frozen screen. One of which is the soft reset (holding down the power button for 10 seconds). This soft reset did fix my problem. There are 2 other options, but these will wipe out your phone and reset everything to factory. Another suggestion is to power down your phone every once in a while, I guess to "cleanse" it and prevent screen freeze.

I realize every phone, including my old iPhone, will freeze every now and then. I can accept that. My Lumia 900 froze only once so far. I'm still a bit nervous because the phone is only 3 weeks old, plus other forums claim this problem occurred before on prior models and was fixed with a software update. I will chalk this up to being a fluke incident, but if it continues frequently, then ... Nokia, we have a problem.

I don't believe in extra insurance/warranty because if you have to get it, then the product is not worth purchasing in the first place.  Consumer Reports also advise against it.  I just hope I never have to regret buying the Lumia 900. I really wanted to see Nokia and Microsoft succeed just to give the consumers more choices than just Apple or Android.