I have also tested a stereo headset with four connector jack (it's from a friend's Verizon phone which has MP3 playback), and it does NOT work with the WIP320. So I have no clue as to what that fourth connector is for.
I got a call from Linksys support in southern California yesterday morning (about two weeks since I spoke with their support staff outside the USA (India I think)). It had been so long I was confused at first as to why they were calling me. Then I told him I still had problems with the headset jack. He said Linksys would ship me a headset that works with the phone?! So I provided my shipping address. If anything shows up I'll post the details here.
He unfortunately had no news on firmware updates or on speakerphone support.
I just purchased the Linksys WIP 320 Wifi phone and I found out that my headset for a Cingular 8125 works with the phone. It did not work at first when I plugged the adapter into the phone, but when I rebooted the phone with the headset pluged into it.....it worked just fine.
And.........best of all...........you can buy the headset on ebay for less than $20.00. It sure beats the Plantronics H261N and H251N headsets that Linksys suggested. The Plantronics headsets cost over $100.00 each.
Additionally, there are two other headsets by Plantronics that may work, too (I have not tried them yet). They are the M206-n1 and the M1330-n1. These headsets are suggested by other WIFI devices that are similar to the Linksys.