75 chars

New answer, new approach.

class M{static void Main(){System.\u0043onsole.\u0057rite("hello world");}} 

It looks like you can hide every character with a \uXXXX sequence. Which makes this thing...
Easy.

Here's my current best that runs anywhere:

158 Characters

using System;class M{static void Main(){Type.GetType("System.\x43onsole").GetMethod("\x57rite\x4cine",new[]{typeof(string)}).Invoke(0,new[]{"hello world"});}} 

.Invoke ignores the first param entirely if its a static method, so you can use 0 instead of null to save 3 characters.

Additionally, a variation that bans the use of the \ character:

234 Characters

using System;using System.Linq;class M{static void Main(){Func<string,string>f=(i)=>new string(i.Reverse().ToArray());Type.GetType(f("elosnoC.metsyS")).GetMethod(f("eniLetirW"),new[]{typeof(string)}).Invoke(0,new[]{"hello world"});}} 

183 chars

Without reflection "cheats"! (but using the deprecated overload of FileStream to avoid using SafeFilePointer)

using System;using System.IO;public class M{static void Main(){new MemoryStream(Convert.FromBase64String("SGVsbG8gV29ybGQ=")).CopyTo(new FileStream(new IntPtr(-11),(FileAccess)2));}} 

Ok, I try my luck:

118 chars

class M{static void Main(){System.Type.GetType("System.\x43onsole").GetMethods()[78].Invoke(0,new[]{"hello world"});}} 

Based on this solution.

Idone

As noted in the original answer, the index of the Write method might be different on your system.