I was tempted to make the title of this blog post be "Did you ever notice how..." After all, how often have you seen an email, forum or newsgroup post, blog post, etc. where the subject was something like that?
There's a reason we have a subject field separate from the body of the message. The subject or title should summarize what the body is about. Having it in a separate place enables the reader to find the items of interest to him. It allows focused searches, effective threading, and indexing of entries. Imagine what a visit to your library's card catalog would be if every book's title was just the first five words of the book!
When you simply dump the first few words of the body into the subject, you're defeating the purpose of both. You're making it harder for me to read whatever collection of messages or items your entry is in: a forum, my email inbox, a blog, whatever. And that means you're making it less likely that I'll read yours.
So use the subject or title to record the subject or title of your post, okay?