Quick start on Arduino for experienced embedded programmer?
As far as I know, there is no tutorials about using Arduino's for, e.g. "multi-tasking, real time response". That is not the focus of the Arduino community.

The Arduino UNO development board is built around Atmel's ATmega328. The Summary datasheet gives an overview of its hardware capabilities on the first page. If you are familiar with MCU development, that should give you a feel for its hardware capabilities. The complete ATmega8/168/328 datasheet is over 600 pages, and describes every feature, with all registers in detail.

The Arduino IDE uses gcc under the covers. So you likely already know how to use that. When you download and install the Arduino IDE, gcc and gnu binutils for AVR will be installed too.

The Arduino IDE has all of the Atmel header files, so much of the code in the datasheet should compile, link and upload. 

The IDE is really designed for beginners. It is to easy to use, but has very few features beyond basic editing, compiling and loading. It does support a serial-monitor to see text printed by the uploaded application, but it doesn't support a debugger like GDB out-of-the-packet. 

However, it doesn't prevent you writing C++ code and header files, and it will build the whole collection of files.

Because the IDE lets you create and edit C++ directly, you can install the IDE on any of the three main platforms, and get started very quickly.

You will probably want to visit AVRFreaks who tend to focus at a lower level than Arduino.

There is also AVR LibC library which points you at the documentation for the AVR upload tool, avrdude, and debugger.

Summary: Install Arduino IDE, do a couple of quick projects to get a feel for the system. Then start writing C++ as normal. Install AVR LibC library for a more traditional library, and visit AVRFreaks forums for more technical help and information.