Inspired by the most excellent by Ping.fm Custom URL plugin by Matt Jacob. This is a much cruder, simpler plugin that does something vaguely similar.
Install through the regular method. If you're not familiar with installing WordPress plugins I suggest you use Matt's excellent plugin, it's much more user friendly. :-)
After install, go to wp-admin/options.php and set these options: * pingfm_default_category * pingfm_status_category * pingfm_microblog_category * pingfm_blog_category * pingfm_author
They should all be single ids. Either category ids or user ids. Just the number, nothing else, easy.
Then head over to ping.fm, setup your custom url as your blog homepage and add this to the end: ?pingfm_token=XXX
Replace XXX with the value of the pingfm_token option. You can change this if you like.
You can also specify the author in the custom url like so: ?pingfm_token=XXX&author=1
Replace 1 with the id of whichever user you want to be the author.
Set the option pingfm_twitter_links to 0 if you do not want #hashtags and @replies to link to twitter.com.