Some of the answers here leave me stunned. Yes, it may not be a problem for some people, specially newcomers to WP7, but that doesn't mean it's not there, or that you have to kill the messenger.
I've done peestandingup's test on my Optimus 7, and it follows my tiny movements perfectly. The 800/900 comparison is even more revealing: the 800 follows the finger, the 900 pursues it. Given the lacklustre commitment to WP7 shown by LG, I doubt my phone has got better internals than Nokia efforts, therefore I'm inclined to believe the lag is solvable via a firmware update.
My current contract ends in July. I know I should wait to WP8 phones, but I also know I'm an impatient man with weak willpower, so I'm considering the 900 (I want a 4,3" phone), but if this lag is the norm, the decision is already made, and not by me. Like dead pixels, I know this would annoy the **** out of me, not just for Dodonpachi, but for any future game or app which requires precise movement.
If you don't notice any lag or you don't think it's a biggie, then I'm glad for you, but please respect any contrary opinion. The last thing this fine community needs is a grow in fanboyism running alongside WP7 grow.
Peace & respect.