If it's anything like the scripting used for the old Flash IDE, then I think it's just straight javascript/ECMAScript. The only real difference is the APIs you have avaialble. I expect anyone who's good with javascript would be able to pick it up fairly quickly.
ExtendScript is very close to regular JavaScript. They've made a few extensions (e.g., operator overloading) but overall the two are very similar. Adobe products include an IDE called the "ExtendScript Toolkit" (ESTK) that provides a nice environment for writing scripts with an interactive debugger.
You can create new menu items in Bridge by creating instances of MenuElement. Set the onSelect property of the MenuElement object you create to be the function you want the menu item to perform when you select it. The Bridge CS4 JavaScript Reference guide has all the details.