I suggest using the stretch method they recommend for button images.
[UIImage stretchableImageWithLeftCapWidth:15 topCapHeight:13]
You can see a working example by downloading Twitterfon's source(it's on the FAQ page). You can see how they code a reusable control for it as well as example images for creating your own bubble.
Edit - Source is no longer available(NDA, possibly)
I put the image up here. You should be able to figure out the rest :)
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