I've never worked with Lua before, but I find myself in a situation where I want to embed a scripting language into a C++/gtkmm app. I've read a number of tutorials involving how one passes variables into the embedded lua scripts and vice-versa.

However, what I've read only seems to pertain to primitive types and C style strings. What I want to know is, is there any way to pass around a Gtkmm widget (less likely) or a gtk+ widget (more likely)? Or perhaps I should be asking if they will even interoperate or both insist on running their own main loop?

Anyone done this before?

There are a number of ways and 3rd party libraries which you use to acheive a C++ and Lua binding.
The simplest is Lunar which is documented on the Lua user wiki.
Swig, although I have seen numerous reports on the mailing list of problems with it when using Lua.
Luaplus, yet this uses a modified version of Lua.
LuaBind uses metaprogramming and Boost to create a binding.
MLuaBind which seeks to do the same as LuaBind yet without the dependency on Boost, instead using the Loki library.
CppLua which I have no real knowledge about.
toLua, which like swig uses a precompilation pass to generate a binding.
SqPlus is a binding which uses a modified Lua called Squirrel which was designed for games by a professional games developer.
CPB which only works with windows using the debug symbols IIRC.
In addition to these I have a binding of my own, yet it is currently broken which multiple inheritance (I really should fix it )