A family of Microsoft word processing software products for creating web, email, and print documents.
Yes, that check box is unavailable unless it is checked (in which case you can clear it) or the current document is in .doc format. It gets checked when you use the Document Inspector to "remove personal information." I said it was a long shot, as I didn't think the "personal information" would include the title.
I'm wondering whether the problem may stem from the difference between Word 2003 and 2007 in that there's no (good) way to duplicate the Word 2003 setting "Prompt to update document properties." Unless you're manually changing the Title field in the document properties (or using a Title content control), I'm not sure how Word would know what the title is. None of that speculation matches what's actually happening, though (since you do see the title, and then it disappears), so I have no explanation.
I can suggest another approach, but it would be slightly more work: use the Title style for the document title and put a StyleRef field (referencing the Title style) in the header. This doesn't change the Title field in the document properties, but it does accomplish your basic purpose.
The best approach would be to use a Title content control; anything you type in that does get written to the document properties, and whenever you update it in one place, it gets updated in any other Title control in the document. I'm sure it must be possible to generate one of these from scratch, but I don't know the trick, so I would be inclined to try copying one from one of the built-in cover pages that include the Title control.