I've been planning on setting aside some money for our children's college savings this year, but was uncertain when the deadline is - the answer is 12/31.
It took a while to search the web and find when the actual deadline is.
There's a great morningstar arcticle that discusses it in detail, but the bottom line is that gift tax rules govern the determination of the tax year a 529 contribution is made.
This means that contributions are considered gifts, tax free up to the eligible IRS gift limit (I think it's $12k per spouse per year, for a total of up to $24k). The gift (in the form of cash), falls into the tax year the gift is given - there is legal debate as to when it's cashed, but it seems to be related to intent.