The iTunes subtitle is the first 250 readable words from your blog post and the itunes summary is the first 4,000 readable words from your blog post. If you use the excerpt field in Wordpress, then Powerpress will use that value as your iTunes summary.
As you point out, the summary or subtitle does not come from your blog post title. If you take a look at your podcast in iTunes, you will see that the subtitle information is typically what you want to appear, as the grid will already display the title of the episode/post. If there is a reason you would want your post title to be the value in your subtitle and/or summary, please email me with a good explanation and if it makes sense I'll add an option to the settings. cio [at] rawvoice .(dot) com