Building with Visual Studio 2012 for .NET 4.0 x86 in release mode, the results differ between running on Microsoft's .NET and mono 3.3.0 (master/6cd4ddc) on Windows with an i7 3820 If a Console.WriteLine call is inserted after the Math.Atan2 call, printing direction, the results no longer differ.

gistfile1.cs

using System;
using System.Runtime.InteropServices;
namespace floattest
{
    class Program
    {
        static void Main(string[] args)
        {
            float currentX = -0.91290748f;
            float currentY = 0.408166558f;

            float direction = (float)Math.Atan2(currentY, currentX);

            direction -= 1f / 30f;

            float dirY = (float)Math.Sin(direction);

            Console.WriteLine(dirY.ToString("G9", System.Globalization.CultureInfo.InvariantCulture));
        }
    }
}