Well, what struck me as odd is this: I was racing the last few days and had those jaggy shadows. I fired up GT5 for another few fast rounds at Tsukuba just now and the shadows are as smooth as silk... No flickering, no jaggy edges, no nothing.
I can only guess what happened, but... maybe the game had to dump some of the data to the hard drive (you know, install it...) in order to smooth the shadows.
Well, I dunno, really, but I don't actually care either. Just hope it stays that way.
Edit: Okay, Tokyo R246 looks perfectly smooth as well.