I have a Sony Ericsson P1i, which doesn't appear to let you use it as a USB mass storage device under OS X 10.4.10. However, thanks to this post by okoshiyasu on the my-symbian.com website, it's now working properly. Here's a brief overview of the fix:
i launched the USB PROBER app, plugged my P1i, gathered the IDPRODUCT info and found out that it was different than the one in the kext (m600 id = 57392 / p1i id = 57490). i changed it using textedit, saved the file, remove the old m600 extension and replaced it with the newly modified one, restarted the computer, plugged the p1i and VOILA! the memory card appears on the desktop.The linked forum thread contains a downloadable version of the modified kext.