Howard,
You are correct that a barcode is a field, and it has properties you can set.
However, the PDF417 in LiveCycle Forms is special. It contains all of the form field data, so re-entry is not needed; scanning the barcode is equivilant to typing all of the field names and field values. It is called a 2D barcode. If you use this type of barcode on your form - the user MUST be using Acrobat or Reader version 7. No previous version of reader can handle this type of barcode. Also, to scan the barcode later, you will need to purchase the "decoder" from Adobe. They are encrypted so that you will need an adobe server to read the data.
PDF 417 barcodes are not this way in all programs - just in Livecycle Designer - Adobe uses LiveCycle Designer with many of its server products.
The other barcode types should work and act as you would expect them to.
We have chosen the Code 128. It seems to print okay on most lasers, and scans with most scanners. I haven't had the oportunity to work with many others, so if someone else would like to offer more information on them, I would also be interested in pros and cons of them.
Cheers,
Samaba