I have been using the same version of FCP for years,(6.0.6) on the same laptop (2.33 GHZ Intel macbook pro). I have not updated any software that would cause conflicts- but all of a sudden FCP rejects:
1.) Pictures in JPG,PNG, TIFF etc. format
2.) Any video with an Alpha channel
3.) Any new titles I generate
To troubleshoot, I have trashed the prefs, duplicated the project, moved the media to a new drive.