There is not a bios update for OSX. If there is, tell me what you did to see that. Why they include the bios update downloads when you select OSX, who knows. But a bios update has nothing to do with running OSX and it is not the first thing needed. The only drivers or OSX specific thing I can see are printer drivers.
Apple says again and again that you will not see OSX on generic PC's. No one wants to take that for what it's worth though. They're not going to secretly start allowing dell to roll out some support for running OSX on PC's when they keep saying this. In fact, it really has NOTHING do to with Dell and requires some significant modification of OSX before it would run on your standard PC hardware.
I'm not saying apple won't go for non-mac hardware and release a copy of OSX for the masses. They've said they wouldn't do things before and then turned around and did them. But you're not going to get any hints of this from dell's support downloads page, that's for sure.
Printer drivers are != running OSX on a Dell.