37signals provides an explanation at the top of the page and a call to action (link to the app store) at the top and bottom of the page. You should consider do

37signals provides an explanation at the top of the page and a call to action (link to the app store) at the top and bottom of the page. You should consider doing the same with your signup form. This pattern is also noted on Google's results pages where they have a search box at the top and bottom.