Plug your phone into iTunes.
Hold down the Sleep and Home button for 10-15 seconds until the phone shuts off, and begins to turn back on. When it turns back on, let go of the sleep button and continue to hold the home button.
iTunes will say, "iTunes has recognized a device in recovery mode, would you like to restore?"
Then restore the phone. There's nothing else you can do. If you back up a lot, then you will be fine. If not, you're gonna lose data, there's nothing else you can do. When the phone has finished restoring, just use the "Setup from Backup" feature instead of the "Setup as a new iPhone" choice.
Wellllll, you COULD try and kick it out of the mode using a program called TinyUmbrella, but it may not work in your case.
Either way, a DFU restore is just easier.
EDIT: Also, if you ended up upgrading the baseband to the iPad one, you're going to have to use a custom firmware to do the restore or you are going to get an error during the restore process. You can find custom IPSW files on google. Just search "iOS 4.3.4 iPad baseband IPSW File" on google and you should get something after some minor searching. This is exactly why people should never upgrade their basebands unless they know what they are doing.....
