Update: 9 Oct 2011. Made some slight changes, still works though.
I've just spent an hour on the phone with Lexmark because my mom wanted me to scan something on my Lexmark X5495 scanner. This is the second time in two month that I have to call Lexmark in order to get my scanner to work. I'm documenting the solution here -- it worked for me.
These steps assume you have already set up your printer/scanner to work with your computer and it has already worked in the past, and now it is saying "Application unexpectedly quit" every time you try to scan. This process takes about an hour and assumes you are comfortable using the terminal.
(1) /Library/Printers/Lexmark/2007/Lexmark\ X5400\ Series/X5400\ Series\ Uninstaller.app
(2) Click on Uninstall
(3) sudo rm -rf /Library/Printers/Lexmark
(4) sudo rm /Library/Printers/PPDs/Contents/Resources/Lexmark*
(5) sudo rm /Library/Receipts/Lexmark*
(6) sudo rm /Library/Image\ Capture/TWAIN\ Data\ Sources/Lexmark*
(7) sudo rm /Library/Application\ Support/Lexmark*
(8) sudo rm /Library/Preferences/
(9) remove usb cable
(10) sudo rm -rf /System/Library/Extensions/LexmarkUSBMerge.kext
(11) sudo rm ~/Library/Preferences/loginwindow.plist
(12) sudo rm ~/Library/Preferences/ByHost/com.lexmark*
(13) Empty trash
(14) prefs > print and fax > no lexmark
(15) restart the computer
(16) Lexmark has deleted this driver from its website as of October 2011. You can find it here, or else through an internet search (it has been deleted from many sites, but it still can be found. Let me suggets keeping a copy for next time you need to scan something).
(17) Go through the install process.
(18) In Lexmark setup assistant, say "My printer is already set up"
(19) plug in the printer (to power and your computer) and scan for printers until you find your printer. (your printer should be on and in copy mode)
(19-a) you'll get a dialog asking you to download software for
x5495, click yes.
(19-b) search your system for an application called Lexmark X5400 Series Center, and launch it.
(19-c) (try scanning -- if it still doesn't work continue down the list.
(20) sudo rm /private/var/log/install.log.*.bz2
(21) sudo rm /private/var/log/system.log.*.bz2
(22) repeat steps (1) to (19)
(23) repair disk permissions using Disk Utility
(24) resart the computer
(25) install, again, the driver you previously downloaded.
If you still are getting the quit unexpectedly message, carefully recycle your all-in-one lexmark center where facilities exist.
Source: 1-800-Lexmark Ref. 1-3520974436 (4 Oct 2010)