While mere mortals will most likely have to wait until May to get at Cupcake (probably even stooping so low as to refer to it as Android 1.5, those commoners), the proud, good-looking possessors of Android Dev Phones can get at the update right this instant from HTC's website.
Naturally, we'd assume somebody's going to figure out how to get these images onto rooted G1s before long, but we wouldn't want to over estimate the intelligence or temerity of the proletariat.