Blurriness in images kills OCR accuracy in any application because the phone can no longer distinguish letter shapes.
If your images are coming out blurry, one possibility is that your camera lens needs cleaning. Try cleaning off your camera lens with a clean, damp, and soft cloth.
Make sure your focus is right. The iPhone camera will try to automatically focus on the card but often misses. Touching the blue square in the middle of the camera view will help ensure best focus.
The most common culprit for blur, however, is a shaky camera while snapping the photo. Try to have your elbows resting on a table or hold your phone against a sturdy object. The best place to take a picture of a business card is from directly overhead, when the card is laid on a flat surface. Remember to try and avoid shadows falling across your card!
You could also use a tripod to eliminate blur. We have have heard from users, and had great experience ourselves, with using the Joby Gorillamobile, a portable tripod for the mobile phone.
NOTE: If you have an iPhone 3G or earlier, or an iPad 2 or earlier, your camera cannot focus well, and you may need to resort to taking pictures of your cards using another camera.