I see that the GD library in PHP has WebP support. What about the standalone GD here?
And libwebp-0.1.2 was released in Apr 1, 2011, will WebP support depends on it?
Yes, php's gd in trunk and 5.4 already uses this library. It was developed using the development tree of libwebp and patches have been sent to google as well (windows support and other similar things).
Btw, it could be fairly easy to merge php's patch into the libgd's tree, if one likes to do it :)