In the past few days a number of reports came in that Windows Live Messenger was blocking the ability to send legitimate urls through Messenger, including most noticeably www.youtube.com.  While there was some sinister speculation as to the nature of this blocking, the Inside Windows Live Messenger blog reports that the problem was caused by a third party partner who handles the blocking of malicious sites:

• There are a number of factors that can be used to determine whether a URL is potentially harmful: number of times a URL is sent, the frequency of the URL being sent, the number of accounts the URL is sent from, manually checking the URL, comparing with other "block lists", etc.
• When a URL is deemed harmful, a block can be instated for a specific URL (i.e. www.<domain>.com/<page>) or an entire domain (i.e. *.<domain>.com)
• This entire process for Messenger is managed by a 3rd party that is a Microsoft partner
• On Friday, Microsoft did not request to block any of the URLs that were accidentally blocked
• The blocks were made by our partner as a result of their process to block harmful URLs
• We are still investigating the specific reason our partner made these incorrect blocks and we will work with them to improve their process for detecting harmful URLs while not blocking safe ones

Everything's all back to normal, now.