I am searching for a development framework to create web applications specifically for use by mobile devices.
What would be the best place to start? I was considering ruby on rails.
Thanks.
It’s also worth pointing out that the actual tool you use for the back-end code won’t make a difference to the end user, all server-side processing is done there (at the server), what the end user will receive will be the mentioned HTML, CSS and JavaScript output which is intended to be run locally. When it comes to mobile web applications, the key issues to consider will be related to design rather than development, therefore frameworks in relation to languages like Python, Ruby, PHP or anything else are irrelevant factors. Fattyjules has highlighted the main points relative to design, but there’s more you’ll want to read up on. 
Whatever you use to build your desktop site can be used to build a mobile site. After all, modern mobile handsets talk the same language, i.e. (X)HTML/CSS/Javascript.
A few things to remember;
- you’ll have to test your site on the actual handsets you want to target, not just emulators
- pages will need to be single column due to limited horizontal space
- pages will need to be light (small filesize) due to limited bandwidth
- pages will need to account for possibly limited input options (e.g. device doesn’t have a keyboard)