Has anyone had success using xcode 4 as an IDE for AVR microcontrollers? Is it possible to have the same amount of integration as the plugin for eclipse?
Yes. I use Xcode 4 as IDE when writing AVR code.
But it only works as a "wrapper" for the avc-gcc command line tools. I have three Xcode targets in the project: build ("make all": compile only), fuse ("make fuse": program fuses) and flash ("make flash": compile and download to AVR). Just select the appropriate target and hit Cmd-B to build.
There is not much integration. I still have to edit the Makefile to set clock frequency, programmer and device model and fuse values. And if I add more .c files I also have to add the corresponding .o file to the Makefile. But at least I can do it from within Xcode.
I have created a minimal project template that will allow you to create a new AVR project in Xcode. Get the file here http://dl.dropbox.com/u/1194636/AVR_Xcode4_template.zip. Extract the archive and put the Atmel AVR® folder into ~/Library/Developer/Xcode/Templates (you might need to create the Templates folder).