I'm having a similar issue - I can upload images using the media manager in WP, and can, presumably, insert them into a post. However, they do not appear when uploaded. You can see that the files are there via FTP, but when visiting the URL where they live, a "page not found" error. e.g.:
http://www.desertsailor.info/wp-content/uploads/2009/04/gwf-fl4.jpg
gives "page not found" though peeking via ftp, you see this image file lives exactly right there. the page on which that image appears shows the image as a broken link.
the ISP folks say it's a problem with .htaccess, which currently reads thusly:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
on my way to disabling all plugins... but any other suggestions?