Though all but forgotten among emergency medicine and hospitalist types it has a prominent place in several guideline recommendations. These are summarized in a recent post at Emergency Medicine PharmD. The relevant guideline documents are linked in the post. Of note:
The Canadian guidelines give it a class I recommendation for conversion of acute onset a fib to sinus rhythm. The a fib duration needs to be known with certainty to be less than 48 hours. (And by the way, enough of this nonsense about diltiazem converting patients to sinus rhythm. This misconception seems to be pervasive. It is driven by the fact that many patients with acute a fib convert spontaneously while on dilt for rate control. True, true and unrelated. Dilt's mechanism is in the AF node and there's nothing there that drives a fib).
The ACCF/AHA/HRS guidelines give it a class I recommendation for pre-excited a fib (WPW) provided the patient is hemodynamically stable.
The ACLS 2010 guidelines give it a class IIa recommendation for hemodynamically stable monomorphic VT (favored over amiodarone).
Keep in mind that procainamide can cause Torsades so watch the QT and be mindful of the K+ and Mg++.