With certain smartphones & technologies you can indeed use the device w/o the cellular service. On my ATT branded Tilt (HTC 8925) I can on demand turn off the cellular radio and only use wifi. Other phones using the Windows Mobile operating system will likely have built-in or third party app ability to do this similarly. Carrier installed software/firmware is another issue entirely. I'm not certain if the same Windows Mobile handset on Verizon may be configured in the same way. You would benefit from popping over to howardforums.com and ask about specific handsets you wanted to buy for this purpose, in addition to the pre (verizon or Sprint version?).
If it's the Pre you're after, I'm not entirely certain you can even buy the newest Sprint or Verizon smartphones directly (unsubsidized, for full retail price) and not activate. Again, Howardforums will be able to answer this. FWIW, I see the Pre on ebay selling brand new for something outrageous like $500-650. Are you really going to spend this much (full retail +) for wifi use-only when a cheap netbook does so much more(?) If you don't buy either a new or renewal contract you will be paying substantially more for the same device.
I'm looking to buy a smart phone but NOT to use it as a phone. I only want to use it to surf the internet and use it's applications like games and such. I would mainly use it at my house where I have access to free wi fi. Is it possible to purchase a smart phone like the new Palm Pre Plus that has wi fi capabilities and not use any cell phone carriers but still be able to use the wi fi feature? I currently don't need a cell phone but will next year so I would like to not activate the phone with any cell phone carriers (meaning not pay anything) until next year but still be able to surf the internet using it's wi fi feature this year.