HTC, Qualcomm, and Motorola. According to the press release, the "Android platform will be made available under one of the most progressive, developer-friendly open-source licenses," and will be composed of a "fully integrated mobile 'software stack' that consists of an operating system, middleware, user-friendly interface and applications." More details to follow.
Read - Google press release
Read - Android mobile OS overview
Read - Google blog: "Where's my Gphone?