I was trying to get rid of subcategories in a parent category (annoying when a post is NOT assigned to the parent category but still shows in it) with the help of a post by Andrew Hall. No luck. I studied the code and dared to comment out another bit that looked “right” and now it seems to work:
in wp-blog-header.php (found in the WP root folder) around line 293 (under “Category stuff for nice URIs”), comment out (= add /* before the code and */ after it) the line

$whichcat .= get_category_children($cat, " OR category_id = ");

Also, updated wpPaginate plugin (by Scriptygoddess) to .1.1.

On WP1.5. this is found in wp-includes/classes.php, on line 413. Comment out

$whichcat .= get_category_children($q[‘cat’], ” OR category_id = “);

On WP1.5.1.(1) these are found in wp-includes/classes.php on lines 393, 396, 420; below Category Stuff and Category Stuff for Nice URIs. Comment out the lines starting with $whichcat .= get_category_children