It is built-in using the now() function, but is that disappearing when you try to add it to a 2003-compatible form? I know the userName() function, which is new for 2007, will not work if you make the form 2003-compatible, thus my inquiry.
Are you not using the forms with SharePoint? You could overcome the IP03 restriction if you were. Office 2003 is about to be 2 versions old, so it is potentially a waste of time and money to be continuing to develope 2003-based forms. I say this more for your powers-that-be than for you, although you would certainly be interested, I'm sure.