Just wanted to share an update on this - so I tried reseting my Samsung SmartHub via the TV settings, this cleared the issue however its more than a bit annoying as you have to reenter your account details for any apps you use (NowTV, Netflix, Amazon...).
So this fix lasted about a day until I tried going back into NowTV this afternoon and again presented with the frozen white screen - nothing happening at all. Rather than attempting to reset my settings again I played with the remote and just hit the "Exit" button on the black Sammy remote. This seems to force the app to actually quit and I was returned to the regular TV channel I was watching. Went back to the NowTV app and it loaded sucessfully second time round - you still see the white screen for about a second but then the little wheel shows up and the NowTV app loads.
My guess is the updated app is a bit glitchy, sometimes the app will hang with a white screen, other times it will load without fault. If it hangs try hitting the Exit button on the remote whereby it should return you to TV mode, afterwards try and load the app again and hopefully it will behave - this seems to work for me so far...