There have been some discussions on the Humanitarian OpenStreetMap Team mailing list about using smaprtphones for tracing/tracking.
I have long been tracing with my phones, first with Symbian, tried iPhone and iPad, and now primarily with Android based phones. The best apps for tracing for OSM are available for Android solely (GPS Logger, Vespucci, OSMTracker for Android).
Unfortunately I have been unable to find a single details source which can say something about the quality of the GPS in smartphones, but have relied on experience, and from experience I can say:
Smartphones are usually pretty accurate, but quality varies greatly and they will often get in trouble in steep and narrow valleys and the like.
You should not trust the accuracy indicator. I have walked for up to two minutes with my phone claiming 4 meters accuracy, while the real accuracy was getting closer and closer to 500 meters. It looked like it used some predictive algorithm to fill in when it had signals but no fix with the satellites. Also, apps (such as GPS Status) will round down the accuracy and not up, so 3,99 m becomes 3.
System bugs can give consistent skewing of position, so keep your system up to date.
Don't use any other source than GPS, they will not be reliable, nor helpful.
Phones will claim to have higher accuracy than possible considering current tech. My Samsung Galaxy Tab would for example claim 1 meter, which is better than any of the chipsets listed on the OSM wiki.
Using a high quality external bluetooth GPS with your phone will improve accuracy, reliability of accuracy and perhaps battery time.
Using external bluetooth GPS with Android smartphone
With standard Android you can use the Bluetooth GPS app to connect your external GPS to your phone. But if have the very excellent Cyanogenmod distribution 7 or newer on your phone you can simply pair the phone and the gps and select it as source in your GPS settings.
This has worked like a charm for me with a Holux M-241, though the reach wasn't very long, and the battery time for the Holux sucks.
Please feel free to transfer some of this to the wiki.