Just discovered the dotnet BigInteger type. Nice use of implicit type conversion and operator overloading.

gistfile1.cs

/// <summary>
/// Primality test of mersenne prime using the lucas-lehmer test
/// </summary>
/// <param name="p">A prime number</param>
/// <returns>true if 2^p-1 is prime, otherwise false</returns>
public static bool IsMersennePrime(int p)
{
    BigInteger mp = BigInteger.Pow(2, p) - 1;
    BigInteger n = 4;
    for (int i = 1; i < p - 1; i++)
    {
		  n = (n * n - 2) % mp;
	  }
    return n == 0;
}

gistfile2.cs

public static void Main(String[] args)
{
    Console.WriteLine(IsMersennePrime(2));
    Console.WriteLine(IsMersennePrime(3));
    Console.WriteLine(IsMersennePrime(5));
    Console.WriteLine(IsMersennePrime(7));
    Console.WriteLine(IsMersennePrime(11));
    Console.WriteLine(IsMersennePrime(13));
    Console.WriteLine(IsMersennePrime(17));
    Console.WriteLine(IsMersennePrime(19));
    Console.WriteLine(IsMersennePrime(23));
    Console.WriteLine(IsMersennePrime(29));
    Console.WriteLine(IsMersennePrime(31));
    Console.WriteLine(IsMersennePrime(37));
    Console.WriteLine(IsMersennePrime(41));
    Console.WriteLine(IsMersennePrime(43));
    Console.WriteLine(IsMersennePrime(47));
    Console.WriteLine(IsMersennePrime(53));
    Console.WriteLine(IsMersennePrime(59));
    Console.WriteLine(IsMersennePrime(61));
    Console.WriteLine(IsMersennePrime(67));
    Console.WriteLine(IsMersennePrime(71));
    Console.WriteLine(IsMersennePrime(83));
    Console.WriteLine(IsMersennePrime(89));
    Console.WriteLine(IsMersennePrime(107));
    Console.WriteLine(IsMersennePrime(127));
    Console.WriteLine(IsMersennePrime(521));
    Console.WriteLine(IsMersennePrime(607));
    Console.WriteLine(IsMersennePrime(1279));
    Console.WriteLine(IsMersennePrime(4253));
    Console.WriteLine(IsMersennePrime(19937));
    Console.WriteLine(IsMersennePrime(23209));
    Console.WriteLine(IsMersennePrime(86243));
}