I can't check right now because the machine I sync with is in Windows, but it looks like it may be in:
~/Library/iTunes/iPhone_Software_Updates
Just in case you want, you can go to that link and put your short name there and then iPhone type/version at the end (it's currently 3G 3.1.3). 😀
file:///Users/*ShortNameHere*/Library/iTunes/iPhone%20Software%20Updates/iPhone1,2_3.1.3_7E18_Restore.ipsw