How does this affect Mac software development? Theoretically, it shouldn't hurt Mac development. The most obvious argument is that with Macs able to run Windows applications -- either in dual boot or in parallel -- software writers know that they could write just the Windows version and that would work on both platforms. However, that argument could (and was) used regarding the old "Classic" OS environment when Mac OS X was first announced, and developers wrote OS X-native apps anyway. Software developers will always listen to their customers, and as more and more people use Macs (especially now that there really is no risk to them with Boot Camp), these same people will begin to demand that applications run natively on their preferred platform.
Microsoft is a good example of this. They've had versions of Office for Classic MacOS and MacOS X, and they're currently working on the Intel MacOS X version. Why? Because their customers demand it. Money talks, and if customers are willing to pay, developers are willing to code.
Does this hurt Microsoft? Not really, if you think about it. Suddenly, if anything, Microsoft has gone from catering to about 90 percent of the market to catering to almost 100 percent of the market. If Apple decided to sell dual-boot machines or, better yet, a version of the OS X that can run Windows in parallel, then people would still have to have a license of Windows. If Apple sold Macs with Windows on it, Microsoft would still make money. Plus, there's Microsoft's Office, which is the industry standard and isn't going anywhere any time soon. Since Office is cross-platform, Microsoft is making money, regardless.
What about using Mac OS X on non-Apple hardware? Will Apple allow that? The answer is no, as long as Steve Jobs is in charge. Don't look for Apple to turn into just a software company. Jobs believes that one of his company's strengths, and what makes Apple highly competitive and innovative, is the fact that it makes and sells the whole widget. Keeping the amount of hardware Mac OS X officially supports (through Apple) guarantees complete hardware and software parity.
However, third-party vendors will have a much easier time writing drivers of their products for Mac OS X because of the Intel switch. They no longer have to worry about writing drivers for a variety of processor platforms and instead of dealing with processor-specific code, they can write drivers for their products for MacOS X.