The following excerpts are from GTS blog:

“Here is the announcement. There are some links at the end of the announcement which you can use to find out more about the API.

Dear Translator Partner,

You are receiving this communication because you are using a Bing AppID to call the Microsoft Translator API service.

This service announcement is to inform you that Microsoft Translator is deprecating the Bing AppID mechanism and replacing it with an access token which supports a more secure standards-based authentication (OAuth).  If you already made this switch, you may disregard this announcement.

To continue using the Microsoft Translator API service without interruptions, you must obtain an access token by April 1, 2012.

To use our service with an access token, please follow the steps below:

1. Subscribe to the service through Windows Azure Marketplace.  Free service is still available at a volume usage limit of 2 million characters per month.
2. Register your application on Marketplace (keep your Client ID and Client Secret confidential).
3. Obtain an access token here.

Will I need to do anything else with my application when I migrate?

Yes, you will need to make slight modifications to your application by following the instructions here.

What happens if I don’t migrate from a Bing AppID to an access token?

When the Bing Translator AppID is deprecated, it will no longer be valid and you will not have access to the service.

What is the cost to subscribe to the Microsoft Translator API service?

We offer a free service subscription tier in addition to higher volume subscriptions at $10.00 USD per 1 million characters.”  More.

See: GTS Blog