Aperture 1.x only supports DNG files originated from cameras for which Aperture 1.x also supports the RAW format. If you import other DNG files, you may see the "Unrecognized File Format" message.
For the list of RAW-supported cameras, see this page.
Aperture 2 offers baseline support for most DNG images, regardless of whether the camera's RAW format is natively supported. For more details, click here.