There are several applications in the marketplace having this feature, maybe one of them list the component used to read the barcodes:
http://wp7applist.com/search/?q=barcode
There's a C# version of ZXing from Google compatible with Windows Phone 7:
http://code.google.com/p/zxing/source/browse/trunk#trunk/csharp
http://sviluppomobile.blogspot.com/2010/08/wp7-zxing-barcode.html
Update this app seems to use ZXing and will release their code:
http://www.phrasememe.com/scanner/
