I have noticed this for a long time since using Google Voice, and found it quite annoying. I'm just now getting around to complaining about it.
I'll have one conversation open, where I'm texting someone back and forth. We'll stop texting for a few hours, and then when that person replies, it usually comes up in a new conversation. This does this on my Droid, as well as when I access Google Voice from a computer.
Any thoughts or suggestions?