Some TTS voices have a different emphasis depending on punctuation.
For example, the voice "en-US, Helen" sounds pretty good regarding this. I'm only hearing a difference between having no punctuation marks after a word, or a period or exclamation mark (I think they sound alike), or a question mark. Also a comma makes it sound a bit different I think, maybe.
But not all voices do this (like Microsoft Anna, it seems). Probably dependent of the default language of the voice too.
Btw, if you got Windows 7 or 8, 64-bit,
this post describes how you may install additional voices for free (including the "en-US, Helen" voice).