ok, i do not use either phone brand or carrier, but i do do this with a motorola e815 and verizon.
first off, doing so (tethering your laptop via phone) can be in violation of your contract. it may not be, but you might want to check. it may also run up massive data fees. if you dont have a data plan, use sparingly or not at all until you know how you will be charged. you could easily spend 100s to 1000s of dollar doing this.
for much better answers on the specifics of your phone and your service, see howardforums.com (they are the macrumors of mobile phones).
to set up a bluetooth modem on the mac is ridiculously easy. make sure your phone is discoverable, turn on mac bluetooth, select add bluetooth device, and pick the mobile phone option. it will let you know if it thinks your phone has modem capabilities. you might have to select a driver, but chances are the sprint pcs driver is the one you want. i am not sure if that is true as well for the treo.
you will then need to "call" your isp, which is your carrier in this case. on their webpage there will be directions for what to do from here. you will need a phone number or code to call, and a generic password.
if this works, great, and enjoy. if not, go to howardsforum.com
i really love the ability to do this and check mail from just about anywhere.