The litter is not flushable if you want to empty the entire box. No one should ever empty the entire box into the toilet.
Clumps of feces and urine are flushable if you use the following litters or their equivalent:
World's Best Cat Litter
These are all made of organic materials that degrade over time.
A clay based litter is NOT flushable at ANY time. Clumps with clay based litter on them will eventually ruin your plumbing!
Sparklekitty, these are the flushable brands that I know of. I have used the Nature's Miracle for nine months with Gizmo and I simply put her clumps into the toilet at once. This disposes of the cat byproducts easily without messing up the pipes. There are more brands but all will state on the box whether they are flushable, and they also indicate that only clumps can be flushed, not the entire catbox full.