Microsoft has announced that it's aware of the problems, and are in a rush to fix it.
According to a Microsoft spokesperson, "We have recently identified an Xbox Live profile data error that results in Error Code '801540B7' for a small number of Xbox Live members. The error instructs users to attempt to re-download their Xbox Live profile and directs them to customer service support for further assistance." The rep added, "We apologize for the inconvenience this has caused to our loyal Xbox Live members and are working to resolve the issue as soon as possible."
Could this have been caused by the Dashboard update? Let us know if you've had the issue with your Xbox Live account.