I'm surprised nobody's mentioned the other option, which the coding standards at a past employer of mine advocated:
if( 0 != x );
Where the constant was always listed first (especially more important with comparisons as they occasionally get typo'd to assignments - the source of many a bug)
It's the style I've used ever since in my C / Perl / C++ / VB.Net since (obviously the point becomes moot in C# whihc doesn't actually allow the if (x) scenario (unless x is actually a boolean, of course).
