It seems the decision is ultimately in the hands of the Authority Having Jurisdiction (AHJ), as long as "equivalent objectives can be achieved" using the device.

National Electrical Code 2008

ARTICLE 100 Definitions

Approved. Acceptable to the authority having jurisdiction.

Authority Having Jurisdiction (AHJ). An organization, office, or individual responsible for enforcing the requirements of a code or standard, or for approving equipment, materials, an installation, or a procedure.

ARTICLE 110 Requirements for Electrical Installations

110.2 Approval. The conductors and equipment required or permitted by this Code shall be acceptable only if approved.

ARTICLE 90 Introduction

90.4 Enforcement. This Code is intended to be suitable for mandatory application by governmental bodies that exercise legal jurisdiction over electrical installations, including signaling and communications systems, and for use by insurance inspectors. The authority having jurisdiction for enforcement of the Code has the responsibility for making interpretations of the rules, for deciding on the approval of equipment and materials, and for granting the special permission contemplated in a number of the rules.

By special permission, the authority having jurisdiction may waive specific requirements in this Code or permit alternative methods where it is assured that equivalent objectives can be achieved by establishing and maintaining effective safety.

This Code may require new products, constructions, or materials that may not yet be available at the time the Code is adopted. In such event, the authority having jurisdiction may permit the use of the products, constructions, or materials that comply with the most recent previous edition of this Code adopted by the jurisdiction.

WARNING

While these devices may or may not be approved by the AHJ, they are not the best solution available. They do not prevent the user from overloading the generator, since the load selection is made by the user dynamically during operation. The better, safer solution, is to install manual or automatic transfer switches. This allows you to pre-select the loads making it a more user friendly solution.

A brief visit to the code indicates that there is a bigger problem, in that the ampacity of 8Ga copper is only 55 at 90C (aluminum 45 Amps) so your wire is too small for a 100 amp service. You need a bare minimum (if everything is rated for 90C) of 3Ga copper or 2Ga aluminum, and probably larger after various derating factors are applied, or if 75C is the limit on any connection. There is a note that may permit use of 4Ga copper for 100 amp in a dwelling unit as a "service lateral" - you'll have to decide how you feel about taking that note seriously. I've linked to a handy calculator that might be instructive.

Ampacity calculator for branch circuits and feeders

Non-pickily, for a wire appropriate to the ampacity of the subpanel, no. If you are using an 8-3 w/g cable, the cable is probably fine, and being a cable would complicate running it in conduit (as opposed to 4 individual wires.)

Pickily, maybe, if the wire is running through an area where it's prone to damage, or through a floor - but in both those cases, the conduit is only needed in the affected area, and is a "sleeve" rather than a "raceway" for code purposes.

If you don't already have the 8-3 w/g cable bought, do compare costs .vs. THHN or THWN (or XHHW, etc) in conduit. Edit: Well, even if you do have it bought, you can't feed 100 amps with it, so shop around for bigger wire.

I have a personal bias in favor of conduit, but that's all it is. I've seen too many scary things in remodeling where rodents have stripped NM cables in concealed spaces. In addition, you are limited to the 60C rating when using NM cable.