I have just written a simple plugin that others running small sites may find of use in dealing with spammers. Instead of locking your site down to private or invitation only, the AutoSandbox plugin allows you to automatically sandbox newly signed up users. You can then have a moderator un-sandbox them if they are legitimate (you will need to use script/userrole.php to establish one or more users as moderators to do this).
For big sites, this sort of individual moderation would not be practical,but then again, for big sites, the public timeline is probably not very useful. For smaller sites, where you want to keep spammers out of the public timeline, this plugin should be quite useful.
You can find it here, enjoy!http://www.box.net/shared/jtnbys9jpl