I can create Speech Bubble-style annotations in preview - and easily change the color and font styles, but the text is always left-aligned. Is is possible to center it?
I look at the speech (and thought) bubble feature in Preview as a nice little extra that provides basic functionality. Unfortunately, at least as of 10.7.3 Lion, text justification isn't an option. If your needs (or wants) go beyond the basics you may be interested in comic creation software like Plasq's Comic Life 2.
There doesn't appear to be a way to do that (other then manually centering it yourself with the space bar).