So, update.
I went to a different AT&T store and asked to see the Lumia 900. The girl who greeted me seemed a little surprised that I asked about the 900 and she didn't seem sure that she was allowed to show me one, but she found a coworker who let me look at his phone (she left hers at home or something). Her coworker gave me a black 900 that looked to be in very good shape, which was a relief compared to my experience in the mall store. The screen looked perfect and the device just looked great (although the silver camera band was still kind of scratched up).
So I went about putting it through its paces. The scrolling speed was the same - slower than I was used to and even a little jagged at times. However, I went through all of my normal everyday tasks and the 900 was very quick when browsing around the device. I pulled out my Arrive and did side-by-side tests of everything I normally do, and the 900 smoked my Arrive every time. It was much quicker opening just about everything. Sometimes the devices reacted at the same speed but most of the time the 900 was very very quick.
It was almost unreal to see how much better the screen of the 900 is compared to my Arrive. The devices I used last week were so covered in grime and oil and grossness that I wasn't really able to appreciate the screen properly - the devices I played with today were very clean and the screens looked awesome. The black 900 had the screen brightness set to automatic, so I went into the settings and turned the brightness to high - and wow. I put it next to my Arrive and was just amazed at how ****ty my Arrive looked next to it.
I tried the camera and took a few pics around the store and it still seemed really underwhelming to me. I guess that's just the way it is. Although I'm looking around the web at iPhone 4S sample shots and I guess they aren't that much better.
The scrolling issue still bugs me, though. My Arrive just KILLED the 900 when it came to scrolling through the start menu and the app list. I took Dave Blake's advice and tried to see if I could time my fingers to start at the bottom of each device's screen and scroll up so that my finger reached the top of each device's screen at the same time, but that didn't seem to make a difference. I usually just give the screen a really quick, violent flick with my thumb anyway so the travel shouldn't make a difference.
So: loved the screen, really warmed up to the build quality, was really impressed with the overall speed at which apps loaded, and was generally impressed with the Nokia apps. Camera still seems disappointing and the scrolling speed still bugs me.
And yet I plunked down $100 and pre-ordered a Cyan 900 after I was finished playing with it. Seeya, Sprint.