[_ck_]

Technically tag_heat_map has been depricated in 1.0a2 and should use bb_tag_heat_map instead but I believe the problem would persist since a wrapper is used.

Try one last shot with this:

<?php bb_tag_heat_map(array( 'smallest' => 10, 'largest' => 30, 'unit' => 'px', 'limit' => 40 )); ?>