Cross platform IPC

Why not D-Bus? It's a very simple message passing system that runs on almost all platforms and is designed for robustness. It's supported by pretty much every scripting language at this point.

http://freedesktop.org/wiki/Software/dbus

For C++, check out Boost IPC.
You can probably create or find some bindings for the scripting languages as well.

Otherwise if it's really important to be able to interface with scripting languages your best bet is simply to use files, pipes or sockets or even a higher level abstraction like HTTP.

In terms of speed, the best cross-platform IPC mechanism will be pipes. That assumes, however, that you want cross-platform IPC on the same machine. If you want to be able to talk to processes on remote machines, you'll want to look at using sockets instead. Luckily, if you're talking about TCP at least, sockets and pipes behave pretty much the same behavior. While the APIs for setting them up and connecting them are different, they both just act like streams of data.

The difficult part, however, is not the communication channel, but the messages you pass over it. You really want to look at something that will perform verification and parsing for you. I recommend looking at Google's Protocol Buffers. You basically create a spec file that describes the object you want to pass between processes, and there is a compiler that generates code in a number of different languages for reading and writing objects that match the spec. It's much easier (and less bug prone) than trying to come up with a messaging protocol and parser yourself.