The John Mockridge Fountain, Simon Perry, 2000
The bench mark for this kind of product has been set by the Museum of Old and New Art (MONA) in Hobart. It gives a taste of the kind of advantages that an app-based guide might have over a printed one. Their gallery/museum guide is an electronic device (the 'O') that automatically provides the data relevant to your location. It also features a range of information that you can choose, based upon your level of interest. This ranges from brief details to extended interviews with the artist. There is absolutely no reason why the same couldn't be applied to public art via a smart phone app.
The key question is who the audience for the product might be. Galleries purposefully remove distractions to the art (white walls, library-like silence) and outdoor sculpture shows (such as at Lorne) usually have rural and natural backdrops that have a similar effect. Cities are a different beast. Keeping an audience engaged is always going to be harder with the distractions of the city bearing down upon them. Unless the tour is very localised, tourists to a city are likely to be as interested in the things between the art as they are in the art itself.
Local users may not succumb as easily to familiar distractions, but the are going to be more demanding clients. They are likely to be more aware of the context, history, politics and artists behind the work they are viewing, and are going to want a lot more than names and dates. They are also more likely to be interested in ephemeral and performance-based work that comes and goes within the cityscape, which is much harder to keep abreast of.
Guided walks, although normally very much the domain of the tourist, offer an alternative benchmark. While a guided walking tour does not have the benefits of being ever available, it does have the potential to beat an app hands-down in interactivity. There's nothing like being able to ask questions and engage in discussions with someone who knows their stuff. Our best experience in such things was via SAW (Sydney Architecture Walks). They even do bike based tours now, helping to reduce the impact of distance on the selection of works that can be included in a tour.