If you want to build a mobile app, maybe it would be better to use jQuery Mobile or Appcelerator Titanium to crank out the first version; you can always build native iOS and Android versions if the project gets traction.
Make use of existing services
Unless you have someone on your team who wants to spend their weekend setting up servers and dealing with hosting, deploy your app to PaaS providers like Heroku (Ruby) or AppHarbor (.NET). If you need more flexibility, you can spin up an Amazon EC2 instance pretty easily. Remember that you can always move your architecture to your own hosted servers after the weekend.
Here are some other services to consider, all with free plans for development:
- MongoHQ/MongoLab – cloud-based MongoDB instances
- SendGrid – API for sending emails
- Twilio – Voice and SMS web service