That was part of the plan. User send a message "8:30PM call me: You look handsome today." to the bot then later you will receive a call from the bot. It is like a command-line alarm on IM.
I have developed a similar app before. Users can set up reminders through a message to a bot on AIM, gchat, MSN ...
Then later they will receive a reminder message from the bot.
Big companies like Microsoft and Nintendo are betting big on this second screen technology. deMobo makes it easy for web developers to add this functionality to existing web apps.
The simplest answer is just to double the dimensions of the image then use CSS to make it the right size.
So if you want to create an HD image at 200 x 200 on a Retina display, create the image at 400 x 400 then use CSS to control the HD image down to 200 x 200: img { width:200px; height:200px; }.
The HD images are still 72 dpi resolution.
The tricky part is then preventing non-HD devices from unnecessarily downloading the larger images.
HTML5 browsers only. Mainly targets smart TV market in the future. It works with 3G/4G and wifi. Hey, Geoffrey, interested in making some cool HTML5 apps together?
I have been working on this similar idea for more than a year now. Really glad seeing more people are interested in this. Ping me if you are interested in chatting a little more about the project.