In recent years the mobile phone market has declined in Western Europe, but one area to have bucked that trend is the Smart phone sector which has successfully moved from being a high-end phenomenon into one that is available to all.
As a developer I'm keen to make sure I keep a breadth of all the latest platforms, so below is a round up of each platform and some information on where you can go to learn how to build to target that platform:
Windows Phone 7
Devices should be launched around Christmas and the murmurs are that the launch is going to be huge. You can develop applications for the the various devices using Silverlight, which is the same language you can use to build RIA applications for the web. You can also use Silverlight to target some Symbian Nokia phones. You develop applications in either Visual Studio 2010 or a cut down version which is free. You can learn more by going to: http://developer.windowsphone.com/
The IPhone has somewhere in the region of 75 million users and each of those users download roughly (and this is a estimate based upon zero scientific research) around 4 Apps a month. You build apps in a language called objective C which unsurprisingly is a C style language. It's quite a tricky language to work on and doing simple things like XML manipulation is a lot harder than it should be, but after a few weeks you should be able figure out it's idiosyncrasies. You'll need a Mac to develop for this platform and the tools can be downloaded here: http://developer.apple.com/iphone/index.action
You can build apps for Vodafone 360, but they call them widgets… You build them using HTML, CSS and other web technologies and you can get started here: http://widget.developer.vodafone.com/en/howtobuildawidget
Is a framework that allows you to target Samsung mobile phones, you build apps using Eclipse so if you’re a PHP or java web dev you'll be right at home. http://developer.bada.com/apis/docs/commonpage.do?menu=MC01040000&mtb1=&mtb2=
To develop for Android you'll need to know Java and the recommended IDE is Eclipse. Android is a good solid framework with plenty of power, but with great power comes great responsibility… it can be easy on this platform to write an application that runs down user battery life so make sure you familiarise yourself with best practices. Android have a large ecosystem of devices which results in this platform being slightly fragmented so you may need to test across multiple devices if you want to support them all.
A platform I have no experience on, but I'll be looking again when they launch their full OS. For now I'd recommend you go to their site if you want more information. http://meego.com/