I just converted most of my posts to use reStructuredText, using the same plugin my old "webrog" blog used. Because I haven't seen any recent posts that make mention of this, I thought I'd post here about it in case anyone else looks for the same thing.
The only thing I had to do was edit my wp-content/themes/default/index.php file to change the text "Read the rest of this entry" in the call to the_content on line 14 to this instead:
<?php the_content('(more...)'); ?>
Alternatively, you could edit the plugin itself to better handle "more" tags within entries.
Happy reST+WordPress blogging!
Update 2007-09-08: This plugin is now a Launchpad project (see the announcement). The link above has been updated accordingly. If you are looking for a way to download the latest rest.php file containing the plugin, view the files for the project, then click the Download link on the right-hand side of the rest.php row.