It took me hours to finally figure out that PS CS4 will arbitrarily decide whether it will display the adjustment layer thumbnail graphic (which indicates the type of the adjustment layer) or just display the smaller "generic" round circle. As it stands now, the aspect ratio of the document you are working on seems to be the key factor that determines whether you will see the full graphic or just the generic graphic in the thumbnail. When I am working on my images originating from 4x5 negatives, I always see the thumbnail graphics, regardless of thumbnail size setting. When I am working on digital images from my Canon 5D, I do NOT see the full graphic (with "small" thumbnail size). If I switch to "medium" thumbnail size, I then see the full graphic. This is REALLY annoying!!
I WANT TO SEE THE FULL GRAPHIC IN ALL CASES REGARDLESS OF MY THUMBNAIL SIZE SETTING.
Its not arbitary - its consistent. Landscape images (larger y value than x) have the small generic icon. This is a space saving feature, because layer masks also change their thumbnails. You would get a lot of wasted space by showing the full icon regardless of document dimensions. And having enough space in the extremely well implemented and powerful layers panel, is very important.
I think its fair to say that its a valid feature request to have a preference for "always show complete adjustment layer icon" - or maybe as an addition to the layers panel, panel options.