I made the jump from Arduino to the Tiny13 / 2313 without too much headache. I found this page which sent me off on a lot of research:
http://www.evilmadscientist.com/article.php/avrstuff
I got a USBtinyISP from Adafruit and was able to get the CrossPack Mac environment up and running pretty easily.
For the Tiny13 specifically, I found this site was really helpful at deciphering the datasheet:
http://avrbasiccode.wikispaces.com
I went the C route, and didn't jump into assembly for this, and was able to get a lot of the basics figured out.
Give it a shot!