There must be a workbook level option that I don't know about.
In one workbook I get a green triangle when I enter the following.
A B C
1 2 =sum(a1:b1)
if I change C1 to say =sum(a1,b1) the green triangle goes away.
If I open a new workbook, and reenter the demo, neither formula gives a green triangle.
I have been using Excel for 10 years and Excel 2010 for many months -- most of my workbooks do not act like this.