Actually the question of enforcing such agreements have still (as far as I know) yet to be fully resolved - i.e, is the price tag an 'offer' or is it 'an invitation to treat'. it is an issue (amongst other issues) that can't be resolved here - only the courts or parliament can do that, and many of us could write an essay about it.
So far, many online retailers (most famously, KODAK) have stood by their error and given the goods as advertised for the price advertised. I guess they have done so out of fear, because normally, if an error is made and it is unilateral and unenforced (as in this case), then despite the fact that, in truth, no 'agreement' or 'consensus' has been reached (an essential element in the formation of a contract), the price "offered" is what the buyer will pay.
Excellent article provided below by Delia Venables explains it far better than I ever will. Enjoy!
http://www.venables.co.uk/n0203mistakes.htm