What tecnology should i use? Currently i am thinking of creating a web application with Silverlight.
I don't believe silverlight is available for mobile platforms. Depending on what you mean by cross platform, you could try J2ME or BREW (but, be warned, you have to jump through lots of regulatory hoops to get BREW apps deployed). If you're okay targeting a specific (if popular) platform, you could look into iPhone / Android / Palm WebSDK / Nokia (in Europe).
Alternately, you could write it as a web app in something like .NET on the server side, and just deliver the app as web pages over HTTP/WAP.
If you go with a web application, you can use MyMobileWeb.
"MyMobileWeb is a low-cost, modular, open-standards-based, open source software platform that simplifies the development of top-quality mobile web applications and portals, providing an advanced content & application adaptation environment."
You can find some nice examples in the wiki.
You don’t necessarily need to use flash or silverlight to create a very contemporary/animated website… I would suggest using ASP.NET framework because of the strong visual studio tools. Also learning CSS and javascript to create amazing animations or graphics.
Web applications designed for mobile devices are generally pretty awful and useless (and not even necessary for the iPhone, which handles regular web sites fine). If I wanted to create something that was actually useful, I'd write it with the .Net Compact Framework (C#), which thanks to Mono would let you cover Windows Mobile and the iPhone (and Blackberry soon, I think, along with Android).
Phonegap is an Open Source cross-platform framework for running HTML and Javascript applications on mobile devices. It uses the native APIs and does not need a web server. Applications are cross-platform but can access device hardware like GPS, sound, and accelerometers.