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It's in your face but unless you've done some A/B testing, you'd be surprised what can affect conversions! :-) FWIW, I have an e-mail newsletter myself at http://rubyweekly.com/ and the way it is right now is the best I've got it to convert. Every time I removed more from the page, the better the conversion, and unsubscribe rates are low (about 8 in total in the last month). Of course, flat out "lots of subscribers"…
Your landing page looks great, but if you want to increase conversions even more you should try putting the subscribe form above the fold. It took me about 5 seconds to figure out how to subscribe.
Either he already changed the site or you using some insanely low resolution, it's above the fold for me even on my laptop screen.