forcing value of conditional tags doesn’t work in 2.6.2

I have been using the method of manually setting the value of conditional tags like is_home after calling query_posts() in the sidebar, like this:

query_posts('some_custom_query');

$wp_query->is_home = true;
$wp_query->is_category = false;
$wp_query->is_archive = false;

this doesn’t seem to work in 2.6.2 anymore. is_category doesn’t change its value. just to be sure:

echo(is_category());
$wp_query->is_category = false;
echo(is_category());

does output both times “1”, when it should output “0” the second time.

is this a bug? is there any workaround?