One of the great features of Bridge is the ability to do a Find based on the metadata in your files. I generally try to add all keywords and other metadata all lowercase, except for proper names. But you may not know if the text you are searching for is upper or lowercase. Luckily, if you search for a word or phrase in any of the individual metadata fields in Bridge CS3, the search is not case-sensitive. However, if you select the "All Metadata" option, the search is case-sensitive.
For example, if you have some images with "Horse" with a capital "H" in the description field, if you search for "Description contains horse" (lowercase h) it will still find the image. However, if you search for "All Metadata contains horse" it will not find the image. You would need to choose "All Metadata contains Horse" (uppercase h) for the image to be found.
I can't think of any logical reason why Bridge would be intentionally designed this way, and have reported this as a bug to Adobe.