As I found out earlier this morning, installing another carrier's OS on your BlackBerry isn't as easy as downloading and deleting the vendor.xml file -- now you have to delete both vendor files. It seems that in one of the updates to BlackBerry Desktop Manager, the vendor.xml file is not only located in one folder, but two. In the past you simply had to delete the lone vendor file and you were well on your way to a clean install with either BlackBerry Desktop Manager or App Loader. Now however, you'll need to dive in a bit more before your PC will see the OS - keep reading and we'll tell you how to seek and destroy (ok, find and delete) those pesky vendor.xml files.
Now this obviously is only for Windows PCs since you can't install a new OS from a Mac.
Next, make sure you can view hidden files and folders in Windows Explorer (Tools > Folder Options > View > Show hidden files and folders)
Then, depending on your version of Windows:
Remove the vendor.xml file and you're done! Now fire up Deskop Manager or App Loader and you should be able to find the new OS and install away!