SERO plan… ur better off contacting sprint CS and try your luck with them. It may be a pain to get, as from what i saw ont he SERO thread on here, but it only took me one call to get it. As far as the PDA, i have the 6700. All in all, i love the phone, and i love that sprint offers unlimited data transfer with “power visoin” which would be inlucded in the SERO plan. There are some glitches with the phone. Freezes sometimes which a soft reset will fix. A few other bugs here and there. I am on my 2nd 6700 seeing that the display took a shit on me a few weeks ago. I don’t know much about the treo, but that is was reviews are for 
good luck with the purchase