There's no actual announcement, but according to the current members page on the IDPF website both Google and Apple joined in July 2010. I'm still looking for the press release.
I'm surprised on 2 counts. I thought Google had joined some time back, and that Apple would never join. Shows what I know.
P.S. No, Hell has not frozen over. Amazon still haven't joined.
About the IDPF
The International Digital Publishing Forum (IDPF) is a trade and standards organization dedicated to the development and promotion of electronic publishing and content consumption.
The work of the IDPF promotes the development of electronic publishing applications and products that will benefit creators of content, makers of reading systems, and consumers. The IDPF develops and maintains the EPUB content publication standard that enables the creation and transport of reflowable digital books and other types of content as digital publications that are interoperable between disparate EPUB-compliant reading devices and applications.