I have scratchbox (armel target) with gcc 4.6.2 and latest binuils which I used for compiling thumb2 binaries (Qt, gtk, ...). Newer libstdc++ should be copied to n900, but it seems there are no problems caused by that.

EDIT:

I compiled gcc and binutils outside scratchbox (statically linked) and setup a new target using /scratchbox/compilers/(gcc462_dir)

And so far the only new library needed is libstdc++