I have had several hits in the last few days from people who did Google searches along the line of "Does Susan Boyle" have Down syndrome?
The most I can find in regards to Susan Boyle and disability is that she suffered some oxygen deprivation at birth which left her "slightly brained damaged".
Here is a beautiful quote that she said about disability:
“I was slightly brain damaged at birth, and I want people like me to see that they shouldn’t let a disability get in the way. I want to raise awareness - I want to turn my disability into ability.”
Source: I Want to Make My Dead Mum Proud
So the answer as far as I can find is that she does not have Down syndrome but she does have a learning disability.