in some previous threads people have discussed adapters from the Treo's 2.5mm headset jack to the standard 3.5mm headphone plugs. My position has been that the best solution would be a 2.5mm male plug with a short pigtail cable to a female 3.5mm jack, in order to provide the most degrees of freedom of motion to reduce strain on the Treo's jack.

as in this TreoCentral store item:

However, I never could pull the trigger on purchasing it from TC because of the high shipping costs ($5) relative to the size and price of the item.

Anyhow, I found a couple of alternative items and have tested them to work fine, so I though I would share the part numbers here for future reference:

Garmin Jack adapter (2.5mm to 3.5mm)
Part # 010-10344-00
(for use with Garmin Rino series GPS+FRS units)
example: item at Amazon
This item looks completely identical to the picture shown for the TreoCentral item.

HP/Compaq PDA headset adapter
HP Part # 342357-001 or SPS Part # 342295-001
This item is slightly different. The pigtail is shorter and has two separate wires with connected insulation rather than the smooth, round insulator of the Garmin/TreoCentral one.

Anyhow, both work fine and it was more convenient throwing this into a larger free shipping order for me than making a special order for just this small item.