This has been a frustrating part of the Speech to Text feature in the Droid Incredible however, I think I have a fix for it.
I start out saying the word "period", and then start speaking my sentence. A period appears first and then the sentence starts with a Capital letter. At the end of each sentence, I say Period, Question Mark, Exclamation Point, etc. Seem to be doing the trick for now. LOL. All my messages will start with a period.