Yes, advances are 30% down, but there's no guarantee that advances will improve when things ease again: publishers had already been tightening the screws for some years before everything collapsed, either directly by offering a smaller advance, or inderectly by holding out for more in the way of territorial and sub rights for the same advance. They're unlikely to go back to bidding more if they can get away with it...
And let's face it, if the average advance for a first novel was £9,000, and it's now £6,000, (divided in three or four payments, minus agent's commission), the difference between then and now isn't exactly going to be the difference between millionairehood and penury, is it. And if you're already published, and depending on advances for some of your income, then you probably can't afford to not publish, both for the money, and for keeping your name out there and your current books in print in the wake of new ones.
Publishers still need new books and new writers: they have to put product through the system. If they want yours, they'll pay for it. Where the recession really bites is in the marginal books - the run-of-the-mill box-ticking commercial stuff, the weird, the crossover, the off-the-wall. Some of those which would have been published now won't be. And maybe, in a year or two, some of them will again.
As ever, the more publishers actively want your book, the better a position you're in. If they don't, not much, then you're not.
One other little warning about the effect the recession is having on writers: it may encourage publishers to try to find grounds to cancel contracts that no longer look feasible:
http://www.observer.com/2009/books/note-authors-make-your-deadlines
Emma