By post_type do you mean category? If so, I think you can do this using the custom permalinks settings in the wordpress admin panel.
%category% is the one you want. Note from the wordpress codex there is a warning about performance if it s at the start of the permalink - I haven t used this, so something you may want to look into:
%category% - A sanitized version of the category
name (category slug field on New/Edit
Category panel). Nested sub-categories
appear as nested directories in the
URI. Starting Permalinks with
%category% is strongly not recommended
for performance reasons.
EDIT - If however, you are looking for post type, as in post / page, then this will be more tricky. It isn t one of the built in structure tags, so you may be looking at having to add this new structure tag, which would be a change to wp-includes
ewrite.php (and reapplying with each new wordpress patch). There may be another way to do it with mod-rewrite, but heading beyond the scope of my knowledge I m afraid