This has probably been answered elsewhere, but the Lollipop logo font is simply called 'and'. It has two typefaces, light and black. It was made bay a company called A2type specifically for Google. It's license makes it non-redistributable but it can be extracted from the Androidify apk under framework/res/fonts.
Unfortunately, the font is not available as a standard ttf to be installed as a system font on Android. and-light is available only as an .otf. and-black on the other hand is a ttf font which can be installed on Android, but both font typefaces suffer from an extreme vertical offset that causes text to be off center or cut off throughout the interface. In order to make the font usable, someone would pretty much have to copy the glyphs onto a new font with proper spacing in order for it to look correct, which would be a considerable undertaking. Which really is a shame, because and is a very attractive font and would make the perfect compliment to the new Lollipop UI.