Great post. It perfectly summarizes all my problems with captcha and the fact that there's not yet a good solution. I think that using computing power to recognize spammy submissions and weed them out (based on time it takes to complete the form or the language used in the post or the attached link) would work well for our purposes but ultimately we have to reduce/eliminate the benefit of spamming in order to stem the tide at the source.
As always, new fads, discoveries and creativity have at their heart basic principles that are sometimes forgotten or overlooked.
Mobile should be part of your creative toolbox, but not necessarily the first one you pick up.