// all environments
app.configure(function()
{
    app.use(express.static(__dirname + '/public'));
    // ...
});

// NODE_ENV=development only
app.configure('development', function()
{
    app.use(express.errorHandler({ dumpExceptions: true, showStack: true }));
    // ...
});

// NODE_ENV=production only
app.configure('production', function()
{
    app.use(express.errorHandler());
    // ...
});

app.configure('development', function() {
   // configure stuff here
});
// becomes
var env = process.env.NODE_ENV || 'development';
if ('development' == env) {
   // configure stuff here
}