Whenever a new item is added to a list, a small icon shows up that says that this item is new.
If you want to disable this, or change how many days this should be displayed, you can do this from stsadm using this command:
stsadm.exe -o setproperty -pn days-to-show-new-icon -pv 0 -url [Your Virtual Server’s URL]
Source: Microsoft KB 825510
Insomniacgeek's Blog 
Is there a property setting when creating a list that allows me to set the days-to-show-new-icon to zero?