Try these posts.. It will help you i think
http://navarra.ca/?p=786
http://spitzkoff.com/craig/?p=108
This will automatically use Google Direction API and will fetch the points in between the locations i guess..
Anyhoo see this blog http://laurilarjo.wordpress.com/2010/10/23/using-google-directions-api-and-drawing-routes-in-iphone-mkmapview/
