If you are not getting a data plan as of yet, I say that cause once you have the treo you most likely will change your tune, you can pretty much just go and get it from which ever carrier you prefer.....they are all the time running deals....
I think Cingular, which I have, changed their data all around and now is much more expensive than sprint last time i checked, so you might want to go with sprint just incase you change your mid about web access.
If you already have an existing contract with someone you might just see if they have an upgraded discount. The unlocked unbranded are still fetching a good bit of money but I find that if you are getting a new carrier and contract there are some real nice deals....
http://www.younevercall.com/
for instance has it for 179.00 which is a far cry from my 575 I paid through cingular....The problem with cingular they have some issues with unlocking all BT capabilities.....DUN is disabled so you will not be able to use as a mdeom for laptop....
Hope you find what you are looking for...its a great phone.....really great!
I have had 8 or 9 smartphones and this one by far is the best!