extracting italics in XML

.. does InDesign mark this in the document  in any way as an Italic font? ..

Alas -- no.

But a perfectly good reason for Adobe not to have done this, is that you can have a Light Italic, Regular Italic, Semibold Italic, Bold Italic, and Black Italic -- all in the same font! Eat that, Microsoft Word! (More in general: the entire Windows font setup is rigged from the start to only "allow" Bold, Italic, and Bold Italic font variants.)

You can still use search-and-replace, to search for an italic variant of your current font and replace it with an XML tag -- CS4 allows a tag in the Replace field (but not in the Find field!). But you have to specify the exact name of your current italic variant(s).

For Times New Roman and Minion, italics are simply called "Italic"; but for Helvetica, it's called "Oblique", and that's what you have to search for. Searching for "Helvetica Italic" does not work. For ITC Garamond Light, it's called "Light Italic"; and for Myriad Pro, well, myriads of its variants are called "weight-such-and-such Italic".

A document has a property "fonts", which lists all used fonts, so perhaps you could loop over this and determine by the names in there which ones to tag; for example, if the name contains "Italic", "Oblique", "Slanted". Be aware that Frutiger and Univers use names like "46 Light Italic", so it may not be a simple one-word string.

(It also leads to another interesting point: if you encounter a "Bold Italic", how do you know if it's been used as a simple local attribute "Italic" in an otherwide Bold text -- such as a heading --, or if it's actually been used to mark Bold Italic text inside regular text? In the latter case, you would need an XML element "Bold Italic" , in the former just plain "Italic". Perhaps you ought to check the default font of the paragraph style ...)