Not in InfoPath 2003. But you can set the font to webdings or wingdings,
which would somewhat mask the password being typed in. Refer to this article
I wrote: http://aspalliance.com/784

You can also use a field in the task pane to enter a password. Then you
would be able to mask the password with asterisks (*). Refer to this article
on the InfoPathDev site:
http://www.infopathdev.com/howto/tutorials/default.aspx?i=e86c3d0c7300432ab02143
bb776223b1
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S.Y.M. Wong-A-Ton

There has been quite a few discussions about a password control in InfoPath.

Microsoft's issue is that it doesn't make sense because the values are stored in plain text anyway--of course this doesn't account for the reality that you could bind the control to a secondary data source (which is not stored).

One solution is to create a task pane. Since this is HTML, you can do whatever you want. Your user would login (or whatever) in the task pane, which could then switch views (or whatever).