Steve Jobs announced today that over the next year, Apple is going to move its Macintosh line over to have Intel Inside. According to Jobs, Apple has been leading a "secret life" for the past 5 years -- each version of MacOS X has had an Intel version and many of its popular internally developed programs have had versions compiled for an Intel version.
For users who purchase Intel-based Macintoshs starting mid-next year, they will need to either get upgraded MacOS X software that has been recompiled for MacOS X for Intel or run it through a compatibility layer that will run software via emulation. It won't be as fast as it would have been natively but with most high end applications expected to be ported over, it will mean that most software for MacOS X should run acceptably no matter what.