As you say, the practice of growing specific plants next to one another to repel or attract insects.

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Note added at 17 mins (2008-05-20 10:52:17 GMT)
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I should say it isn't just relating to insects but a whole range of beneficial reasons for planting some plants next to others; The best example is the 'three sisters' planting method where corn, beans and pumpkins are grown alongside one another. The beans grow up the strong stems of the corn and also fix nitrogen into the soil which the other two plants benefit from. The pumpkins act as ground cover preventing weeds and also stop the top soil from drying out. It is a traditional planting method in many civilisations...