Yea i would also like to know if such a app exsists. Coming from a Treo 755p thats exactly how convos were grouped. The IM style is awesome and would love it if i could have SMS the same way on my BB's.
So the question is, Do they make an app that makes your SMS into balloons....... I don't believe so. I think if you are looking for an iPhone style, go with the iPhone. If you want superiority....... Umm.... Blackberry