Not a lot is really ever said about Apple and Linux. Sure, and I heartily agree, Apple stole from BSD to make a profit (Maybe now they'll change their licensing scheme?), but that in the end helps Linux users. There are some pros and cons to this though. I'll start with the pros.
First off, if Apple does succede Microsoft as number one dog in the computing industry, making ports for applications should be easy. There will be little excuse not to provide a Linux build, as directories are layed out similarily enough, along with having similar applications, all available to each other. Also, if people get used to the Mac filesystem layout, it will be easier for those who come from the Windows layout to see how the filesystem works. But there are more cons I suppose than pros.
Well, judging Apple simply by their current marketing scheme, it's easy to see that they'd not be a kind competitor. Lockouts would be most definate, and most PC marketers would be out out business, as Apple refuses to let anyone else make hardware other than them. So, this would mean you'd be forced to use Mac hardware, and it would no doubt you uncommon features to make it difficult to install anything but OSX on.
Overall though, I think it would do the Linux community good it Apple only had 30% of the market share. This would give developers excuse to make things for Mac's, and when it's made for a Mac, it's easy to build for Linux. This would allow the Linux market share to grown, and it would then have it's own set of developers wanting to produce.