Changing the item row background color might be the best way to implement this type of feature, however there are some complications when using this method.
If the entire row background is colored and full row select is turned off then the entire item background isn't properly colored when selected.
If full row select is on the problem doesn't occur, but then you can't tell the background color of the items being selected.
Next idea was to color just the modified date column row background, this works fine except when the list is sorted by date, when sorted by a specific column the background is defined by windows and I do not see a way to custom draw it per row.
The original suggestion was to bold the text for the row but this conflicts with the marked list feature.
I welcome some suggestions and feedback on how this feature should work, right now I just don't see a clear and ideal way to implement such a feature.
The visual display is the only thing holding back this feature, the backend code is as simple as
if item.date > site.lastlogin then apply_special_row_style(item)