It appears that YouTube has been quietly changing its logo across the various social networking platforms it uses to promote its content. Additionally both the Android and iOS apps now feature the new logo. We're still not sure though, if this is an official permanent logo update as YouTube.com still features the old bubbly style logo.
The new logo as seen above appears to be more Windows 8 friendly and looks like it would fit in perfectly with the live tiles on any Windows 8 home screen. Additionally the logo does appear to more closely align with other Google-owned services such as Chrome and Drive, which both are quite minimalist and features double drop shadows. I have sent a request to YouTube for more information and will update this post as soon as I know more.