The recent jump in autism cases is reason for concern, especially if these trends continue.  A just released study by Cornell University (Waldman, et al) reveals a significant correlation between autism rates and young children who watched TV frequently.

Previously, vaccines were a leading suspect, but numerous studies have failed to show any definitive link between autism and vaccines. 

Gregg Easterbrook of Slate Magazine points out that further study is needed to clarify the link between autism and television:

"Research has shown that autistic children exhibit abnormal activity in the visual-processing areas of their brains, and these areas are actively developing in the first three years of life. Whether excessive viewing of brightly colored two-dimensional screen images can cause visual-processing abnormalities is unknown. The Cornell study makes no attempt to propose how television might trigger autism; it only seeks to demonstrate a relationship. But Waldman notes that large amounts of money are being spent to search for a cause of autism that is genetic or toxin-based and believes researchers should now turn to scrutinizing a television link."