from the article Matt linked to above:
Don’t BGS And PSA Slabs Have UV Protection Built In?
No, actually, not really. BGS says they add UV resistant material to their slabs. But when asked how much, or to what level they are protected, the answer was “as much as we could add without yellowing the slab”. Clearly (based on the evidence above), if there is any protection it’s not very much.
PSA, on the other hand, makes no claims of UV protection in their graded cards. That means it’s your job to protect your cards.