Global warming and clouds

Clouds act like a blanket. During the day they block the sun and therefore the earth doesn't heat up as much. During the night they reflect the heat back to the earth and therefore the earth doesn't loose as much heat. So would they prevent global warming? NASA wonders that too:

For example, if Earth's climate should warm due to the greenhouse effect , the weather patterns and the associated clouds would change; but it is not known whether the resulting cloud changes would diminish the warming (a negative feedback) or enhance the warming (a positive feedback). Moreover, it is not known whether these cloud changes would involve increased or decreased precipitation and water supplies in particular regions. http://isccp.giss.nasa.gov/role.html