Singular when you are talking about them as a general category. Plural as a rule if you were to refer to several of one category.

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Note added at 2003-05-28 23:18:51 (GMT)
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Example of the singular use as a general category:

\"Soaring along on your own (and having chance encounters with curious birds!), getting a bird’s eye view of a stunning landscape- that’s what *aerial sport* is all about.\"

Example of the plural use from the same article:

\"Physical fitness is top priority for anybody who’s keen on aerial sports. You’ve got be in good condition, and not overweight. Suffering from vertigo is obviously a no-no when it comes to aerial sports.\"

In the latter case the writer envisions certain specific sports from the general genre.