If you import a multilayer Photoshop file (.psd) into Aperture, it may not look the way you expected. Aperture doesn't recognize layers, so it may only show one layer from the Photoshop file or nothing at all.
To prevent this, open Photoshop and choose Preferences > File Handling.
In the resulting dialog, choose Always from the Maximize PSD and PSB File Compatibility pop-up menu (as shown above), which will make Photoshop embed a flattened version of the image into the file that Aperture can use.
This issue is resolved in Aperture 2.x.