The photos and instructions on this thread are excellent. I have 3 BMW's and do a fair amount of mechanical maintenance. However, when I looked at my e90's battery I thought "what the $*%$? So I sought out this thread and found this to be pretty easy.
I bought my battery from the "Stealership" for $161 bucks but I value an exact fit. I was a little bit alarmed when I fired up the car after the replacement to find the brake light and 4X4 lights on. They went off before I got out of my driveway, however, and everything seems to be fine. All I had to do reset was the clock.
A number of folks have talked about having the "Stealership" reset the battery management system. I think I'll mention this the next time I'm there. It seems that with the relative complexity of the charging system that it would be "smart" and not require this. It smells of another elitist line of proper BMW maintenance. Sometimes it requires creativity to justify the labor prices they charge. Too many customer's at the dealership don't know jack about cars; they drive BMW's because of the image and are easily snowed.