I have used Google Earth a number of times in maths lessons. It has mostly been to make use of the 'ruler' tool and provide real life links to the lesson.
The first use I've found for the ruler tool is for providing real life links to calculating area and perimeter. The ruler too can be used to measure the dimensions of the school playground, a football pitch (see below), a child's garden, the courtyard of Buckingham palace or even The Pentagon, anywhere really! This can be either as a whole class on an interactive display, with everyone calculating areas and perimeters based on the measurements taken or as individuals taking measurements on computers or hand held devices. As the ruler tool measures accurately it offers an opportunity to discuss rounding. Also, if calculating the area and perimeter of part of your school the activity can be carried out virtually on Google Earth and then in 'reality' by going out with trundle wheels and then comparing the two.
The other time I have used the ruler tool is when converting lengths. I've usually done this converting between centimetres, meters and kilometres, but it can also be used to practise converting metric to imperial or vice versa. The example below shows how the length of one side of the Pentagon can be measured and then converted into various different units of measure. The original measurement was taken in meters. I'd then ask the children to convert this to centimetres, kilometres or feet, inches etc., if covering converting to imperial measurements. Once the children have their answers one click of the button can quickly show them the measurement they should have arrived at. The measurement of 'Smoots' always causes some amusement and a good teaching point about less well known units of measure for length.
These are just some ideas I've made use of for using Google Earth in maths lessons. If you have some ideas of your own I'd love to have some comments below.