Update: Xbox LIVE on Windows Phone seems to be working fine now.
It seems we are not quite through the service outages just yet as many, including myself, are experiencing issues when trying to access Xbox Live on Windows Phone. If you try and access your XBL account, you may receive the following message:
Could not sign in to Xbox LIVE.
Please ensure airplane mode is off, you still have cellular data or Wi-Fi connection, and your account settings are up to date.
Microsoft is aware of the issue (via their Twitter account) and is working to correct the problem:
Hopefully the services will not be down for too long and judging by how short my own outage was yesterday, Microsoft should have us up and running very soon. Stay tuned for more updates.