So, restart I did, and, what do you know? I went back to the Print & Fax Preference tab, and I saw that the printer listings (both my original one and the new one it created) had functioning Scan tabs. Now, the good news: I realized after installing the driver that I hadn't restarted. My 'original' Lexmark 5400 Series printer was listed still, but the Scan tab went to a screen that continued to say Scanner not Connected. First, the bad news: it installed a 'new' 5400 Series printer (in the Print & Fax Preference tab under System Preferences) but with very few options, and the tab for Scan was gone. With my hopes up, I downloaded and installed the driver. I found that on Lexmark released a driver file called 'Lexmark 5400 Web Installer 10.6.dmg'. For months, I haven't needed the scanner until today, so I dug around Lexmark's website. Lexmark CS (or lack thereof) was of no help.

I had the same problem (printer worked, but scanner and card readers did not) with my Lexmark X5470 after updating to Snow Leopard.

Download lexmark 5400 series driver