Wow...that's a new one.... I have never seen that before... I have seen the boards, but not the cake...lol
Geez...I have seen a stamp at Michael's that is wood grain... Maybe load it with food color and just be careful that the edges match... A stencil, maybe? I can't imagine trying to just pipe it... I will be interested to see the ideas that folks come up with on this one!
theres a wood grain tool that you can buy at wal-mart in the hardware section (where the wood staining stuff is). i think they even used one on ace of cakes... well anyways- its a rubber curved stencil type thing. all you have to do is put some food coloring on it... and drag the tool across the surface, rocking it back and forth for the knotted look. i had one years ago- but used it for actually making imitation wood grain with wood stain... and it was awsome. you will have to do it on fondant... because it will not work on buttercrea.
In the Martha Stewart wedding cake book..there is some instruction how to do that. It uses clear acetate and two different colors of chocalate with the wood grain tool found in the paint section of a hardware store.
actually i saw this on ace of cakes last night. If you have a large sheet of fondant, you can spread liquid food coloring on it. the coloring will make the fondant kind of sticky, then use a brush to brush horizontally. they did this to make a desktop look for a typewriter cake.
Could be a chocolate transfer (tho, I haven't found one with this particular pattern) or an edible frosting sheet.