I picked up an Arduino yesterday, and I was quite dismayed by how much work it took to build/flash from the command line.
After some hours of Googling, though, I finally found an up-to-date Makefile, and it didn't take too much to make it "just work" with the default install of Arduino.app on the Mac.
To use this Makefile, simply drop it into the project sketch directory, then run make then make upload. You may need to tweak some variables (eg, it assumes Arduino.app is located at /Applications/Arduino.app), but that is fairly straight forward.
Get the Makefile from: http://gist.github.com/273821.
Just a note: I haven't had much time to tinker with this system yet, so it may have some serious issues... If so, drop me a comment below.
Also, leave a comment if you want me to let you know if (when?) I update this.