That's pretty easy to do...
First insert a new "text" type column into your plan. Right click on the column to open the "customize field" and then choose rename to make it something like "Status". Under "Custom Attributes" choose formula and enter your criteria and give it a numerical resultant value. Then click on the "Graphical indicators" button and define Test to be "equals" then the value, such as 1, and then choose the icon you'd like to dsiplay for that particular value.
For example, one of the formulas I have is...
"IIf([% Complete]<100 And [Finish]<[Current Date],0,IIf([% Complete]<75 And ([Finish]-14)<[Current Date],2,1))"
Hope this helps you out.
Zen