The BitConverter class has a field IsLittleEndian which, according to the documentation:
Indicates the byte order (\"endianness\") in which data is st
The CLI standard does not proscribe any particular Endianness, so if you want your program to be portable, you should not depend on a particular byte order... unless of course in scenarios where a particular byte ordering is required, such as with some data exchange protocols (thanks to user The Moof for pointing this out).
From the CLI Annotated Standard (p.161) — Partition I, section 12.6.3: "Byte Ordering":
For data types larger than 1 byte, the byte ordering is dependent on the target CPU. Code that depends on byte ordering may not run on all platforms. [...]
I suspect that you saw a hard-coded value for IsLittleEndian in Reflector because when you downloaded/installed the .NET Framework on your machine, that particular installation package was targeted at a particular platform (e.g. Intel x86, which is Little Endian).
I could thus imagine that there are other installation packages of the .NET framework that have IsLittleEndian hard-wired to return a different value, depending on the platform that particular installation targets.