This gets tricky. The sentence as you state it is correct (She is more beautiful than I). As Zooey, correctly states, if you were refering to spoken English, the sentence would be "She is more beautiful than me," even though it is strictly incorrect.
However, consider these two sentences:
"Jane likes Tim more than I."
"Jane likes Tim more than me."
Both these sentences are grammatically correct, but mean two different things because of the IMPLIED sentence.
The first sentence has the implied meaning: "Jane likes Tim more than I (like Tim)."
The second sentence would mean: "Jane likes Tim more than (she likes) me."
If the implied sentence uses me as a subject (the first) then you would use "I"; if it uses me as an object (the second) then you would use "me".
I like the Chinese way of doing it better.
As for Jane, I still don't know if I ever want to talk with her again. ;-)