The Apple Developer Connection (ADC) has posted an article titled The Sound of Opportunity Knocking: The Audio Units Community Takes Off. The article discusses how to build Audio Unit plug-ins for special audio effects in applications like GarageBand, Logic Pro, and Soundtrack Pro. From the article:
First, the framework provides a complete foundation for writing robust audio plug-ins, because it lives at the OS level rather than the applications level. In addition, it frees you from worrying about applications compatibility; most any music program running on the Mac can host your Audio Unit plug-in, and many in fact have this capability already. That great custom sound effect or unique digital instrument will work seamlessly with GarageBand, Logic Express, Logic Pro, Final Cut Express, Final Cut Pro, Soundtrack Pro, Motu Digital Performer, BIAS Peak and many others.
The article is available at Appleis ADC web site.