Also, a link to www.something.com (without the http://) by definition implies that you're linking to a file called www.something.com in the same directory as the page the link is on. "something.com" is just the file extension for a file named "www".
[edit]And of course the fricken phpBB auto-links my intentionally malformed links. Who'da thunkit? [/edit]
phpBB will pickup anything with http:// and www. in the proper format.
But if you just post something in a subdomain (like wtfe.site.com) it won't be right unless you put www. or http:// in front.
Scripts don't know subdomains, they're all geared to look for www. or http:// as a prefix. To look for subdomains is not really worth the effort to code it.
[/edit]
