Apple iPhone 4 (CDMA)
Iphone to Boost Mobile
It isn't that it can't be done for other phones, it's just that no one cares enough to go to the considerable trouble.
i would imagine sprint will be getting it soon enough. with att and vzw having pretty similarly priced plans, people would really jump ship to save some coin with sprint. only speculation of course.
to get it on a sprint contracted plan, you would have to put an esn on it that sprint already has in their system. as far as i know, that isnt legal.
boost im not sure of since its relatively new. but i know a lot of people that have non boost phones active on a boost account.
Visual Voicemail with Verizon - $2.99/month
source ( http://wirelesssupport.verizon.com/features/c alling_features/visual_voice_mail.html?t=3 )
Cellular service with Verizon: working
Source: Everybody is Los Angeles