We started with CodeIgniter but decided to just go with raw PHP on a LAMP stack, in the name of speed. Hosted on a VPS over at Rimuhosting for now. We might upgrade to Cassandra and dedicated servers if this ends up gaining traction but that's far down the road.
Mobile support is absolutely in scope. I think we'll start with just a m.poly variant and, given enough demand, offer free apps for various platforms.
Right now we're a two-man team but we're sailing smoothly - let me send you a PM with some more details