Mobile development, what language?

It's hard to anseer this question without knowing more about your skills, the target mobile devices you're aiming at and the server's platform but if you're a Windows aficinado, VB, C++ and C# may be thr ight choice for you. MSDN has plenty of samples and online tutorials, especially in C++ and VB, which you can use. J2ME may also be considered but remember that it's a bit more restrictive (to may taste at least) because everything has to be confined to this sepcific platform, so you can expect interoperability as long as every participant adheres to this tool. Another option that you want to consider is BREW, which is a C++ based platform.

It's manufacturer dependent, but I think BREW and J2ME are the most widely supported platforms at present. Personally, I'd opt for BREW because it's more efficient and doesn't require a wholesale virtual machine installation. As you may have realized, there's not a single platform that all manufacturers support so perhaps you can write yoru code in C++, and then be able to port it with some tweaks to both Symbian and BREW. It's not a trivial task but at least you'll be covering two large categories of mobile devices.