It indicates the designer's insecurity. Problem sites are recognizable with or without under construction signs. Good web designers do not use the under construction warning. A complex background may make a beautiful art piece, but it can make it impossible to read text. Select a background that is not busy.
Muted colors work best.
If no background is chosen, a battleship grey default color results. This is unattractive and creates an unfinished appearance. So, either a background color, or GIF, should be chosen. In this same vein, designers must remember to use a contrasting color for the text. If a light text color is used on a light background, it will be hard to read, as dark text on a dark background will be hard to read as well.