I just finished developing a Windows Phone 7 Barcode scanner based on the ZXing library. You should be able to find it in the marketplace under PhraseMeme Scanner or get more information from
PhraseMeme Scanner
There are two big issues with the WP7 platform right now when it comes to barcode scanners. First there is very limited access to the camera. Basically all you can do is launch the camera app and have the user take a picture. This means you cannot setup the photo for micro focus, change the resolution, or use a video stream for real time scanning. Second the framework automatically downsizes photos to around 1 Megapixel, so while you might take a 5MP photo it will be processed at 1MP.
I had to do some image resizing to get ZXing to process the photos correctly, and while it works pretty well on my phone in good lighting conditions it might not work well on other phones.
If you download the PhraseMeme app you can take photos and try processing them. If the photo doesn't process correctly try saving and emailing it to the support email address and I'll see if I can improve the scanning algorithm. The next version of the app will make this process much easier so that I can get real world data and hopefully greatly improve the scanning performance.