I just compared two JPEGs exported with LR 3.6 and LR 4.1RC. The tags you mentioned are exported neither by 4.1 nor by 3.6. These tags are only present in the normal Exif metadata, but not in XMP. Which version of LR 3.x did you use before? Perhaps Adobe changed something already in the later versions of LR 3.x. If this is the case, you have to use a script or tool that is able to read the Exif metadata directly.
In the XMP metadata in the file exported by LR 3.6, I see only crs (camera raw), aux (e.g. serial no.) and some other XMP namespaces, but no exif or tiff.
P.S. I viewed the metadata of the exported files with exiftool. File format was JPEG (with "quality" setting, not with "limit file size", because apparently there are still some XMP differences between these two modes).
I suspect this may be due to Adobe adhering to the latest recommendations of the Metadata Working Group. If I recall correctly, certain information that natively exists in the image file is no longer replicated in the XMP block. You may need to adjust your scripts accordingly, as the MWG approach is likely to be the standard for the foreseeable future. I believe the change occured around the LR 3.5 timeframe.
To follow up some more...
The latest MWG spec may be found here. A discussion of the issue is on page 22; specifically:
"However, this document changes this earlier XMP guidance and recommends that Exif and Tiff device properties only be mapped to XMP in the case the file format does not support Exif natively."
The MWG spec dates to Nov 2010, and as I noted earlier, Adobe adopted this guidance sometime last year for LR and, I believe, Photoshop as well.
That sounds like the answer then. I have been using Lightroom 3.2 until I upgraded to 4 so that would also fit with your observations. I must say that's a really frustrating change as it makes the information a lot harder to retrieve. I'd certainly appreciate it if Adobe gave us the option to include the data for backwards compatibility, but I guess that's unlikely.
Thanks a lot for your help,