An easier way is to use Radial Blur, set the blur method to zoom. You need to tweak the amount to get the effect you want. You can also shift the centre of the blur effect to the area you want to be in focus. It's far more realistic than a motion blur which is too even all round to simluate the lensbaby effect.
Here's the photoshop action that you can try. Scroll down to Lensblur action and download it. I've tried it before but although it does somehow mimic the effect of a Lensbaby, having the real thing is more than fun.