29
You can take your existing C knowledge when using Arduino.
The purpose was to allow artists/non-programmers to get started easily with hardware programming and tinkering, so the 'Arduino language' is just a wrapper to simplify development.
It should be a lot easier for you, as a C programmer to use Arduino. The documentation isn't lengthy at all, the wiki is nice and the people on the forum are enthusiastic and helpful.
Arduino is C-like and extremely easy to pick up. They have abstracted away from doing things like reading and writing into peripheral registers for doing basic tasks.
One look at some example code and the Arduino reference and you'll be up and running in no time if you've actually done real C on any other platform.