David Letterman grilled Lindsay Lohan like a cheeseburger today ... firing off question after question about her upcoming stint in rehab ... until Lindsay finally shot back with a dig of her own.
Letterman broke the ice by asking, "Aren't you supposed to be in rehab now?"
Lindsay responded, "Don't you watch anything that goes on? What are you a tabloid now?"
Lohan eventually told Dave what he wanted to hear -- she'll be entering rehab on May 2 and believes it's "a blessing and not a curse."
Letterman kept pressing ... asking if Lohan believes she truly has an addiction problem -- and that's when Lindsay hit back.