This is the new security featrure in Tiger which attempts to prevent malicious programmes from being run at startup. I don't know what application is featured in the dialogue box - but if you don't trust it clicking disable will prevent it from starting up at start-up.
It is indeed the Lexmark Printer driver. I installed it recently for a Dell printer (A920), which is a Lexmark copy. I also received the alert at startup.
I have a lexmark Printreo X1150. I got the same message after I installed it on my Imac G4 running 10.4.1 I clicked on Fix. Ever since, whenever my Imac tries to go to sleep the printer does something that Immediately wakes the computer back up. Anyone have any Idea on how to fix this?