Question: On a 64-bit machine:

Can a 64-bit application call a COM server (out proc) which is running in 32-bit process?

What about vice versa? (32 bit app calling 64-bit COM out proc server)

Thanks!

Answer: 
The whole point of out-proc COM is that the two processes interact via RPC (usually LRPC), so it doesn't matter at all which bitness each of them has.

We used COM+ surrogate for forcing 32-bit in-proc components into a separate process for the only purpose of consuming them from 64-bit clients many times - with all necessary stuff for marshalling present it works without any effort.
This may be relevant [MIDL: 64-Bit Porting Guide](http://msdn.microsoft.com/en-us/library/ms810720.aspx).

Basically what it says is that if you pass pointer types (`IUnknown` etc) it's OK but if you *cheat* by passing a pointer disguised as a `DWORD` you may face some problems.