With regular Google map, you can specify multiple destinations in the same URL. I was hoping to be able to create a bike route using Google maps and then e-mail it to my iPhone and thus display it in the iPhone’s Google map application. But no such a luck. The iPhone Google map application cannot handle multiple destinations. In the regular browser based Google map you can directly encode a route in a URL, e.g. here is route from San Jose, CA to Los Gatos, CA to Salinas CA:
Unfortunately iPhone’s Google Map Application is unable to process this kind URL. It looks a like a bug that hopefully will get fixed in some future upgrade.