There seems to be some open source libraries around.
In Ubuntu, there are the libdecodeqr0 and libdmtx0 packages with corresponding libdecodeqr-examples and libdmtx-utils packages containing sample programs. Then there's also QRcode.

It wouldn't be too difficult to write a small Maemo GUI for any of these libraries (certainly those with python bindings). I might even do it myself if I find the time