Mbed.org is almost as easy as the arduino , but it's a professional tool used by some companies, and it is pushed by ARM(powerful company) as the dev tool for the internet of things .
Arduino is more of a hobbyist tool.
If you interested , there's a step-by-step book teaching the mbed:
http://www.amazon.com/Fast-Effective-Embedded-Systems-Design/dp/0080977685
After you finish the book, do a project , and you will be set.
In general using mbed(mbed.org) will save you lots of dev time(they offer lots of libraries). A good beginners book on mbed and embedded development is :
http://www.amazon.com/Fast-Effective-Embedded-Systems-Design/dp/0080977685
Another good book is :