I wish to write an iOS application that has to download data from the website using Wordpress as content managment system. What' s the easiest method to do that? thanks!

What type of data ? If this sounds like publish, page , comment you need to use XML-RPC : http://codex.wordpress.org/XML-RPC_wp

Uhm, one solution is by using the wordpress export function (wordpress-admin/export.php). It gives for you an XML file that you could parse.

Once you are drenched in to the account, you are able to problem an easy get. Or, you are able to download youOrwebpages via XML-RPC, as already noted!http://codex.wordpress.org/XML-RPC_wordpress