From an Organic Gardening site, http://www.organicgardening.com/featureprint/1,7759,s1-5-22-1055,00.html
This may simply indicate wear and tear, an iron or nitrogen deficiency, or that the grass is going dormant. Before attempting to treat one of these pest infestations, get your soil tested to determine whether organic soil amendments will green up the grass.
What Causes It
Billbugs lay eggs in grass, and their larvae feed on stems, causing yellow circular patches that appear drought-stressed.
Fast fix: Add organic matter and water deeply in spring to discourage billbugs.
Prevention: Reseed or overseed the lawn with resistant endophyte-enhanced grasses. (Check seed package labels for these special varieties.)
Chinch bugs suck the sap from grass, which causes yellow circular patches to develop.
Fast fix: Water the lawn well for three to four weeks to a depth of 6 inches.
Prevention: Reseed or overseed the lawn with resistant endophyte-enhanced grasses.