Similar experience to Johnavia. Once on a Bae146 on a delayed late night flight MUC-MAN. The a/c was about 50% loaded and the entire back 10 12 rows were moved forward - apparently for reasons of weight and balance. The aircraft had been about to divert on the inbound to MUC and the airport had opened at the last minute, so perhaps they were carrying loads of extra fuel or somthing?
Anyway, the front of the cabin was really crowded and people kept 'sneaking' to the back - by landing everyone was spread out again!
I know the 146 has a rather pronounced nose-down attitude on approach (due to the lack of leading edge slats). I wonder, does this type of thing make that aircraft more difficult to balance than others, or are longer aircraft (say, 757's) more difficult than the more 'stubby' ones?