i have the same issue though it's not a big one of course.
i too noticed the same thing a week or two ago but since 1) my eyesight is poor 2) it's not that unreasonable for the preview pic to be of a low res until you zoom, i just brushed it off. however, i did think 'wait a minute, i swear preview pics was a bit clearer before' so i guess this confirms it's not just me.
btw i have my photo albums set in grid mode; the low res thumbnails is when you press to view an individual pic.
p.s i'm on stock 2.1, not unrooted or debranded etc + it's not 'low' res as in 'omg i can see the pixels!' but they're a tiny bit blurred out. it's subtle but noticeable since i already subconsciously thought of it.