How is this effect achieved?
- This is just "painted" in photoshop
- A custom shaped aperture is used
The non uniformity of the bokeh leads me to believe that the background has been Photoshopped. Exactly how it was done is anyone's guess - but the sharp lines through the bokeh may indicate a liquify or similar brush that "grabs" the colors and drags them. I do think a bit of manual brush work was done, in addition to some filters.
However, do be aware that some lenses, like lomography's Petzval lens, come with interchangeable plates that change the shape of the bokeh (ovals, star, etc). It's also possible to mess with the bokeh shape by cutting a hole in a piece of paper and placing it directly up against the front element of your lens.
Edit to add:
I stand corrected. @void_ptr has pointed out that the Fractalius plug-in does exactly this effect.
Given this new info - the creator most likely applied the plug-in to a duplicate layer of the image and then masked the original flower back into the now Fractalius-ed background.