This is a port of the ZXing library to Silverlight for Windows Phone 7. At the moment the ported version is 7.1. I know that ZXing 2.0 is already available. So, the port of the newest version is planned as well.
The changes that were done to adapt ZXing's C# code to Silverlight are described in details in the following article - Silverlight for Windows Phone 7: How to use ZXing 7.1 in Silverlight.