Voice is one of those areas that's very context-dependent and subjective, so my generalisations below will not work for everyone. But I would suggest the following is generally true:
Purpose-built devices such as Polycom/Spectralink Wi-Fi phones are still the best-performing clients. But the gap is narrow enough that in some situations today, our customers are using SIP clients on smartphones in similar roles, with good satisfaction.
There are two main areas where smartphones with SIP applications fall short today, QoS and handover performance.
While the mobile OSs offer the hooks for applications to signal their priority to the OS, and hence transmit their frames with the correct WMM priority, not many applications developers use these hooks. I frequently see packet traces where voice & video traffic from smartphones does not have the correct priority set. When I last compared applications in detail maybe 18 months ago, for instance, the Counterpath Bria was the only app asserting voice priority on iPhone, and even Counterpath wasn't quite there on Android at that time. Not setting voice priority won't be a noticeable problem until you encounter congestion or difficult RF conditions, of course.
The second area where a smartphone will not match a purpose-built device is in inter-AP handover (roaming). This is because the underlying chip (most Android and Apple devices use the same family) is not optimised for enterprise WLANs, although things are improving. In my experience, maybe 70% of handovers are clean and crisp, and the other 30% can give rise to outages ranging to several seconds. That's while carrying a call around a 'normal' enterprise WLAN at a fast walking pace. Static or near-static clients wouldn't see any bad effects of course, if you don't move around, the phone doesn't need to hand over. If you have the time and inclination, the attached is light fireside reading on the subject of handover performance. While the QoS tagging is invoked by the application, the handover performance is generally a function of the platform, chiefly the Wi-Fi chip and driver, and the app won't affect this aspect of performance.
We expect things to improve significantly after the Wi-Fi Alliance unveils its Voice-Enterprise certification programme, expected in mid-2012 and we start to see Voice-Enterprise compliant APs and client devices. We are also seeing more smartphone developers interested in making their products work well with multimedia traffic in multi-AP environments.
In the short-term, I would encourage you to try out a pilot with a handful of devices and see how it goes. We have some customers who are very happy with current performance, while others are waiting for something a little more solid. But it's very easy to get a few devices going and test them, and your subjective experience and your users' views are the ones that matter. While Android has a native SIP client now, which I've found quite good, third-party SIP apps are readily available for both Android and iOS.
Good luck, please report back if you try it!