Someone on another forum asked how it's done, so I thought I'd also post the inner workings here. Here's the breakdown of how my scroller works. This is totally done in actionscript with no actual tweening.
1)From an XML file, I generate a list of thumbnails to be loaded, and arrange them in a row
2)for each thumbnail, there is actionscript that detects how close the mouse cursor is to itself, and if it is closer than a certain number of pixels, it will ease/expand dynamically.
3)there is a movieclip on Enterframe that constantly spaces each thumbnail a certain number of pixels apart, after taking into consideration the size of each thumbnail (as they could have expanded).
4) another on enterframe movie clip makes sure the entire scroll bar is always centered on stage.
I guess the hardest part is finding the right math formula.