We travelled into Tibet from Nepal recently (with a Tibetan guide that we were told was necessary) and, although we were told beforehand that we would need to leave a huge deposit at the boarder to get the bike in, when we arrived, the officials saw the carnet and wanted to stamp it instead of using the deposit. They were, however, pretty confused about the whole deal and we had to direct them about what to fill in and what not to.
We have heard that they are in the process of beginning to recognise carnets and that by next year this could be in force properly. We're not sure how true this is, but it worked for us last June at that particular boarder...
Jim and Lisa