Most mobile development occurs under either Symbian OS, Windows Mobile or Java ME. However, there also a few opportunities to work with proprietary operating systems. Up until recently, there was very little information on how proprietary technologies worked. However, more and more information is becoming public domain.
For example, Motorola have recently created opensource.motorola.com where you can find kernel source code, open source SD-MMC drivers and the source code for a JAVA test framework. If you would prefer to work with Nokia devices, there’s also gnokii.org which allows you to program against some of Nokia’s proprietary external interfaces.