I searched for this in the forums site but didn't see an answer, so I thought I post one here, for what it's worth. I use both the i-Go car and home power adapters and found a tip that fits the Treo 800w. The tip number is A97 (mine is marked 'Rev B') which is for some Motorola devices. It works on my phone and charges it just fine on the car charger.
http://www.igo.com/detail/IGO+6600697-01