As the story goes, Nvidia has infringed a patent regarding to PCI prefetch that OPTi holds and although OPTi no longer makes PC chips, they now license intellectual property. The problem in this case is that Nvidia didn't come out on agreeable terms with OPTi and has to remove the PCI prefetch feature from its chipsets.
The problem with this is twofold. Firstly, it means that just about every single Nvidia chipset from the nForce 500-series and onwards is affected and as such, PCI prefetch has to be removed from all retail boards and every single BIOS upgrade that contains the PCI prefetch code has to be removed from the web. This also means that all Nvidia partners have to release BIOS updates for every single Nvidia motherboard from the nForce 500-series and onwards.
For the consumers, this means that they'll get very poor PCI performance, so if you're using any PCI cards in your Nvidia based motherboard, don't upgrade to the latest BIOS, as it will reduce the performance of the PCI bus. This isn't going to be popular with a lot of users out that that have already upgraded their BIOS, as this "feature" has already been implemented by some motherboard manufacturers going back a couple of months.