The National Weather Service has officially called last weekend November 7th and 8th 2009 an Indian Summer.

Indian Summer

According to the American Meteorological Society's Glossary of Weather and Climate, Indian summer is defined as; a time interval in mid or late autumn, of unseasonably warm weather, generally clear skies, sunny but hazy days, and cool nights. At least one killing frost and preferably a substantial period of normally cool weather must precede this warm spell in order for it to be considered a true Indian summer. It does not occur every year, and in some years two or three Indian summers may occur. 

Based on this definition, the weather in Chicagoland last weekend qualified as an official period of Indian summer. Another period of sunny and unseasonably mild days is forecast for the end of this week.