I replaced the 3.5mm plug on my Sony headphones with a 2.5mm plug I picked up in Radio Shack and it works perfectly with my 600. In addition, when the headphones are plugged in, the phone mic is still active, so I can use the headphones instead of a hands-free. My speakerphone gave me a lot of echo sometimes but I haven't run into that problem using the headphones.
Has anyone else tried this, or if someone does now, let me know how it goes. Its easy to do and the plug is only $2.