Stacking a large number of JPG images (several hundred or over a thousand, as in a timelapse capture) is stalling out on "gathering information". Only way out is to force quit and restart. It worked normally in LR 3.6.
LR 4.1 RC (same issue in 4.0)
Mac OS X 10.7.3
Manual stacking. All images I've tried it with are "normal" JPEGs (about 4000 x 3000 px) and have tried with several sets of images and during several sessions after system restarts--stalls out every time. Worked normally for me in LR 3.6.
Europe, Middle East and Africa