I weigh MUCH more after a workout! I weigh-in at home and try to weigh-in 2 days after a good workout... This is specially true if you do heavy weight training...
If you consistently weigh in after exercise, wouldn't it be accurate, though? I mean, you'd maybe weigh more, but week to week you'd be comparing apples to apples, right? Shoot, women fluctuate so much week to week, anyway. :P Howie and I weigh in at our doctor's office on Friday mornings, before we go swimming.
Garden wife -- I think you're absolutely right, that if you weigh in at the same time every week, whether it be before or after a workout, eventually you'd see differences going down on the scale if you are eating properly...I always weigh in the morning before my shower...