It looks as if similar issues have popped up before, but I couldn't find anything I felt was completely "in my pocket".
I'm currently running 9.14.6992 off a Core 2 Duo iMac w/ 4 GB of RAM and 10.5.8. My downstream speed is easily 4+ Mbps if not a full 5.
Netflix playback is simply jerky, dropping a couple or a few frames just enough to affect smoothness of pans, etc. Quality ranges, but that could be the transfers. I'm also finding that within 20-30 minutes I'm getting the "your speed has slowed" error and either have to wait a while or reload the flick.
Obviously, I'm not experiencing these issues with in-browser playback...