If you haven't heard the news yet, you will soon. It's all over the blogosphere and on the writing forums. Yesterday, December 3, 2008, was one of the worst days in the recent history of book publishing.
Publishing houses are laying off staff, freezing acquisitions, and even closing down or consolidating their publishing lines. The market you may be planning to submit to today could be gone tomorrow.
The best advice I would give anyone who is writing a book? Sit tight. If you haven't queried agents yet, don't. Now is the time to work on your craft or to start a new project.
If you have an agent who hasn't subbed to publishers yet, talk to her/him about waiting until the dust settles. The industry is in a huge state of flux. The last thing you want to happen is to have your book lost in the confusion.
By this time next year, we may be seeing an entirely new model of book publishing. There could be new submission standards, new kinds of contracts, new publishing houses, and new means of distribution.
You can read more about Black Wednesday and what is happening in the publishing industry, here and here.