// test just one category

<?php
if ( in_category('{ID}') ) {
	$phpbaytext = '[phpbay]keyword,5[/phpbay]';
        echo phpBayPro($phpbaytext) . "";
}
?>

// if testing multiple categories (which is more likely)

<?php
if ( in_category('{ID1}') ) {
	$phpbaytext = '[phpbay]keyword1,5[/phpbay]';
        echo phpBayPro($phpbaytext) . "";
} elseif ( in_category('{ID2}') ) {
	$phpbaytext = '[phpbay]keyword2,5[/phpbay]';
        echo phpBayPro($phpbaytext) . "";
} else {
// show some default ads here or AdSense or whatever
	$phpbaytext = '[phpbay]keyword3,5[/phpbay]';
        echo phpBayPro($phpbaytext) . "";
}
?>