When I export a JPEG from Lightroom 3.6, is the white-balance temperature and tint value written in the JPEG file somewhere? Could I later take that JPEG and see (in the EXIF for example) what the WB values I used in the developing process are?
When you export from Lightroom (via the dialog, rather than using a predefined preset), there is a section in the Export dialog called 'Metadata' which allows you to control the amount of detail contained within the metadata. If you set it to maximum (I use LR4 so can't tell you exactly what the options are in v3.6), then the white balance and tint will both be contained within the exported file's metadata.
If you open the JPEG file in Photoshop and view the raw metadata, you will come across the following section:
<crs:WhiteBalance>As Shot</crs:WhiteBalance> <crs:Temperature>4850</crs:Temperature> <crs:Tint>+1</crs:Tint>
This contains the information you need.
To view the raw metadata go to
File -> File Info... and then click the tab labeled
Raw Data. There you can type in
WhiteBalance in the search field, which will lead you to the white-balance information.