I am not signing up for an account so I don't know exactly how the API works, but if I were setting up an iPhone App to work with this, I would use the rails plugin. Rails gives you JSON communication for free (IE no extra work required) http://guides.rubyonrails.org/layouts_and_rendering.html see section 2.2.9. Then use on IOS app use something like: http://code.google.com/p/json-framework/ to interact with that.
Instead of simply sending out a broadcast sporadically, why not choose to create a newsletter. That way, you can send one out each week/month that includes said promotions, offers, site updates, etc. in one place. That way, subscribers won’t get annoyed of you sending out too much content to them, as they can expect to get everything in your regular newsletter.
Now, you can automatically send highly relevant emails encouraging them to buy the product or service they were considering. Customers who received multiple abandoned shopping cart emails are 2.4 times more likely to complete the purchase than those who receive only one followup email, according to Experian. Try sending the first message one day after, a second message 48 hours after, and possibly a third message within three or four days of abandonment.