Read about this app today (some magazine) and i downloaded it and tried it out right away. You use your iphone as a doppler and listen with headphones (it's not a real "doppler" - the app use the microphone to "filter & amplify baby's heart beat sound").
it took us awhile to find the heartbeat and it does not sound as clear as a real doppler, but it definitely worked and made us happy :-).
We even recorded the heartbeat and post on facebook. it's so cool!!!!