for Twenty Ten, you will need to edit loop.php (ideally in a child theme) and change the conditional statement in this line:
<?php if ( is_archive() || is_search() ) : // Only display excerpts for archives and search. ?>example to allow full posts also in the 'portfolio' category:
<?php if ( !is_category( 'portfolio' ) && is_archive() || is_search() ) : // Only display excerpts for search and for archives other than cat 'portfolio'. ?>