It happened
Its called SERO Premium. SERO users can call and for an extra $10 per month, you are able to add any device that Sprint has. Plus, you get Navigation and Any mobile, Any time as well.
Just keep in mind, that if you go for an Evo or an Epic, you will also get charged an additional $10 per month making your total bill to $50 per month.
I switched over to SERO Premium and then picked up a used EVO and I pay $50 per month. Evo is a great phone.