There is a way to use a Single Line text field when creating a column in Sharepoint Services 3.0 and that field behaves as a Paswword field (hiding what the user entered) ?
Thanks in advance!
MSDN - How to create a custom field type
This topic provides an overview of the process of creating a custom field type and defining how it renders on list views and on the Display, New, and Edit forms.
For a concrete example of creating a custom field and defining its rendering, see Walkthrough: Creating a Custom Field Type.
The alternative 'hacky but pragmatic way' is as Vladi suggests using some Javascript (perhaps in a CEWP) to modify the pages HTML.
You would want to find the field INPUT tag and change the type to Password (this will then use * instead of chars).
This post gives you an walkthrough of opening up a New/Edit.asp form in edit mode, adding a CEWP that contains JavaScript and modifying the page. Its not exactly what you want but will get you started.