I'm sorry to have to report that after the initial excitement wore off, the difficulty of having to log into a PC won out over the greatness of the app itself and my Spanish learning halted.
So you imagine how excited I am that Duolingo have just released an iPhone app! I was, in fact, so excited that I was slightly wary of first opening the app, worried that I'd be disappointed. So many sites rush out a mobile app that is vastly inferior to the website version. I needn't have worried as the Duolingo iPhone app is in fact better than the web version with new exercises specifically designed to be easy to use on a mobile.
No more excuses to not keep learning, although it will probably still be another few months until I'm good enough to post in Spanish....
Duolingo are working on an Android app, due out in 2013.