Why? I think Apple is trying to transition to a single brand for their mobile and desktop operating systems. Certainly, many of the features in Lion were inspired by features in iOS. And it would make sense for Apple to want their users have a common user experience between their iPhone, iPad and iMac or MacBook. Steve Jobs said that the Mac and PC have been "demoted" by iCloud to being "just another device." Apple is not content for the desktop to be the digital hub because the desktop digital hub is not always available. The cloud is, so by keeping all of your devices in sync via iCloud, they make it easier for you to own more devices. At least, they make it easier to own more Apple devices. And that only makes sense because Apple is a hardware company.
So the rebranding of Mac OS X to simply OS X is Apple's first step towards a single name for their two operating systems. One day you will see all the devices they sell as being so similar in user experience that there will be few barriers to stop you from buying more of them. And that's the point, removing the barriers that prevent prospects from becoming customers.
What do you think? Why would Apple remove the "Mac" prefix from Mac OS X?
On a related note, in this blog post, Bob Cringely stated that the iCloud's real purpose is to kill Windows. iCloud might indeed be the final missing ingredient that makes the iPad a realistic option for Windows users looking to replace their current PC. After all, when considering a new Windows PC versus an iPad, price is no longer an issue. However, I don't think Apple spends time thinking about killing Windows. I think Apple spends its time thinking about creating great products. Anything else that happens is nothing more than a byproduct of that thinking.
This all may make me sound like an Apple fanboy. I'm certainly a fan of theirs. However, I'm a fan because they have consistently made great products. There are plenty of companies for which I'm a fan for the same reason.