What is available?
Google has released the entire codebase for the Android SDK as the Android Open Source Project. From the bootloader (a minimal reference version unfortunately) and runtimes to the end user applications such as email, calendar and the voice dialer are available for download and use.
Unfortunately there is still only one hardware device capable of running Android but the HTC G1 (currently only available from T-Mobile) has demonstrated some of the capabilities and some of the promise of the Android platform.
Not completely open
The bootloader will most likely be customized for various phone providers. Some of the code will also be stored on the device ROM and will not be reprogrammable without reprogramming the ROM with some kind of jailbreak. Some argue that because of this, some argue that Android is not truly open source. The underlying code is available and open source and I would argue that this is a huge step in the right direction when compared to previously closed mobile platforms.