While playing a bonus feature? Most bonuses are still in standard definition for older titles--perhaps because it's the same features as are present on the standard DVD editions. That, and a lot of older features were shot to standard def: interviews, for example, that were shot before high-def camcorders hit the market. While PS3s will upscale the image, it's possible that it's simply giving you the information of the source material.
This also applies to many concert disks, which were likely shot on standard-def equipment. The only way to turn a standard-def source into a high-def copy would be to upscale before applying compression (VC-1 or H.264, respectively), or to upscale during playback like a standard DVD would.
Otherwise, you can try looking for firmware updates if you're seeing this switch while watching the main movie on a true high-def title.