Unrecognized selector is an exception, and you can set a breakpoint on exceptions.
In Xcode 4, go to the breakpoint navigator, click the +, and choose "Add Exception Breakpoint". Under Exception, choose Objective-C. Under Break, choose On Throw. It should look like this when you're done:
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…