As I reported a few weeks back, there's apparently enough variability in the magnet placement in the 8300 vertical leather flap cases so that about half of them will properly put the 8900 to sleep, and the other half will not. I love the way the case fits the 8900, so on my last visit to Best Buy, I tried about a dozen of the cases on the 8900, and bought one of the ones that consistently worked with the 8900's sleep function. The fit is perfect -- looks good, secure, but easy to get the phone in/out with one hand. Perfect case for me.
If you have the opportunity to try a few, you will most likely find one that works just fine.