Just discovered Arduino, and am very impressed with many things. Especially all the HW and the modularity. Please bear with me and take no insult. I could not find any of this in FAQ.
How does the Arduino development tools compare to AVR Studio?
Can I just use the AVR Studio with Arduino HW? Are *.h files available?
Is there a comparison chart between the two development environments?
Not sure if I am missing some philosophical, critical or mayor difference here.
The Arduino development environment consists of an IDE, high level libraries and a bootloader. It is very quick way to get started.
I have not used the AVR studio much so I can not really comment on usability. You can use the AVR studio to program the Arduino using the ICSP but you may erase the Arduino bootloader (which you could also re-flash with the ICSP port).
You can also use avr-gcc with a Makefile and flash your programs using the ICSP port.
This is what I use most of the time. I compile Arduino sketches with make.