That's about as true for mashups as it is for the semantic web; in both cases, there is not an "official" trust model used across the web, though it's obviously possible for semantic web providers to do what every other API provider does and simply use their reputation. For example, what makes you trust the data you get from any API at all, if not for the provider's reputation? I do agree that there should be a more uniform approach in order to get wide usage of the technology beyond the enterprise, academia, and medicine.
This apparent lack of an official trust model won't be the case for long with POWDER [1] on the horizon. POWDER is on its way to becoming a ratified W3C standard and it makes steps to address the idea of a "web of trust." Who knows what kind of traction this will get in the near term, but I think it's a step in the right direction.