I have loaded my 1.7 world into the latest 1.8 pre-release.
I have noticed that my two-block tall flowers, such as Rose Bushes and Sunflowers started popping up. They were manually planted in my garden. Not all of them popped up, it seemed random. Manually created Tall Grass popped up as well.
Also, some of them were corrupted, only showing the bottom block of the flower. After re-planting the issues seem to go away.

Natural flowers seem to be unaffected, but this needs confirmation