I tested <span></span> in FireFox 0.8 and 1.0. It certainly didn't wrap (my nodelets where wider than my screen, much less my browser window).
Here is a test case:
TheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisitionTheSpanishInquisition
If you view this node directly and you don't see horizontal scroll bars, then your browser is likely wrapping <span></span>.