click on your harddrive, open Applications, open Utilities, in there is a program called Terminal. Click that and it will open a little window, where you can type in the commands. Make sure your iPhone is connected via a USB cable.
At the prompt in the Terminal window type the following
iRecovery -s
you'll get a bunch of text and stuff, at the next prompt type
printenv
another list of text appears, look through these for one that reads auto boot - false
at the prompt type
setenv auto-boot true
then type
saveenv
then type
reboot
at this point your iPhone should snap out of recovery and start to reboot in 3.0 as if nothing had happened.
See also this thread
http://discussions.apple.com/message.jspa?messageID=9922328#9922328
good luck
Failing this, simply attempt to do the download again from a different location using a different inertnet connection, it looks like something is getting blocked by certain ISPs or routers or firewalls or some such (who really cares)
Another alternative if you don't want to do the irecovery thing is you do this instead:
http://blog.mcchouse.com/2009/08/iphone-os-301-update-error-3014.html
good luck