Media Mail as a class is not closed against inspection. Thus, our local post office and (in theory) a few others have been opening virtually all incoming and outgoing Media Mail for the past year (since July 1, 2009). Larger post offices are supposed to do spot checks. The explanation is that this enforcement program may eventually go national.
When non-qualifying material is found inside the package, the recipient is charged postage due for the difference from standard Parcel Post delivery (at this point, no additional penalties are being applied). That difference may easily double or triple the cost of shipping.
The biggest problem for users of Media Mail is that the definition of qualifying items is somewhat vague. "Advertising" is prohibited in material shipped at Media Mail rates, but advertisements are often incidental components of items that would otherwise be considered media.
The article notes that during these inspections, the USPS is also looking for evidence of inappropriate use or reuse of Priority Mail and Express Mail shipping boxes.