Here's my test code to isolate the problem:

open Microsoft.FSharp.Metadata
[<EntryPoint>]
let main args =
  let core = FSharpAssembly.FromFile @"C:\Program Files\FSharp-2.0.0.0\\bin\FSharp.Core.dll"
  let core2 = FSharpAssembly.FSharpLibrary
  let core3 = System.AppDomain.CurrentDomain.GetAssemblies() 
              |> Seq.find (fun a -> a.FullName.Contains "Core") 
              |> FSharpAssembly.FromAssembly

  core.Entities |> Seq.iter (printfn "%A")
  0

All three lets should give me the same FSharpAssembly. Instead, all 3 throw an exception that FSharp.Core is not an F# assembly (details below, re-formatted for readability). Two more clues:

1. Using the core3 method, I get the same error for the test F# assembly itself
2. I don't get the error at FSI after doing #r "@C:\Program Files...\FSharp.Powerpack.Metadata.dll".

Any ideas? I'm using Visual Studio 2008, F# 2.0 and F# Powerpack 2.0.0.0 (May 20, 2010) release on an oldish XP VM, I think it's updated to SP3 though.

(I got the error this morning with Powerpack 1.9.9.9, so I upgraded to 2.0.0.0. I thought that if 1.9.9.9 doesn't recognise F#'s 2.0.0.0's assemblies, then maybe bugfixes in Powerpack 2.0.0.0 would help.)

Unhandled Exception: System.TypeInitializationException: 
  The type initializer for 'Microsoft.FSharp.Metadata.AssemblyLoader' threw an
    exception. 
  ---> System.TypeInitializationException: The type initializer for
    '<StartupCode$FSharp-PowerPack-Metadata>.$Metadata' threw an exception. 
  ---> System.ArgumentException: could not produce an FSharpAssembly 
    object for the assembly 'FSharp.Core' because 
    this is not an F# assembly
Parameter name: name
   at Microsoft.FSharp.Metadata.AssemblyLoader.Add(String name,Assembly assembly)
   at <StartupCode$FSharp-PowerPack-Metadata>.$Metadata..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.FSharp.Metadata.AssemblyLoader..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.FSharp.Metadata.AssemblyLoader.Get(Assembly assembly)
   at Microsoft.FSharp.Metadata.FSharpAssembly.FromAssembly(Assembly assembly)
   at Program.main(String[] args) in 
     C:\Documents an...\FSMetadataTest\Program.fs:line 11
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