What tape should I use to seal leaks in forced air heating system?
Looking at the specifications on the page you linked to, it appears to be made of the same basic materials as other duct tapes: plastic coated fabric with a rubber adhesive.  Premium Grade it may be, but I think you'll run into the same problems -- it'll get brittle and crack with age.  I've used aluminum foil tape where I've needed spot repairs; Home Improvement 1-2-3 says to use duct mastic when installing ducts.

I found this article very useful: Can Duct-Tape Take The Heat?


  Duct Tape Cannot Take The Heat
  
  Although our testing has not been able
  to differentiate amongst other sealant
  products, the data shows that cloth
  duct tape is not a good sealant for
  use in ducts that operate at much
  above ambient temperature. We believe
  this is due to the rubber adhesive,
  but cannot state so definitively. For
  the most part, cloth backing and
  rubber adhesives go hand in hand. Thus
  it is not surprising that the other
  sealant products have not demonstrated
  any of the failure modes we have seen
  in the duct tapes. There are a few
  products that use rubber adhesives
  with non-cloth backing and we intend
  to test these in the future.


To address your side note about Class 1 ducts: from the scope for UL 181 Standard for Factory-Made Air Ducts and Air Connectors


  Class 0 - Air ducts and air connectors having surface burning characteristics of zero.
  
  Class 1 - Air ducts and air connectors having a flame-spread index of not over 25 without evidence of continued progressive combustion and a smoke-developed index of not over 50.