Plenum rated piping
For information - Plenum Rated specifically means that it meets ASTM E84 standard for Flame Spread Index of less than 25 and Smoke Developed values of less than 50. This doesn't mean that the substance won't burn, and it doesn't mean that the fumes aren't toxic, it just means that it spreads an gives off smoke within a certain rate.
If you have to use non-plenum rated materials in a return air plenum, you just need to cover it appropriately - you can meet code by wrapping the piping with insulation that meets the plenum rating (which most HVAC duct insulation does).
Basically check the HVAC system, if the air handler has 2 ducts coming out of it, and one of them is just open above the ceiling, that's called an open plenum return system and you have to meet the ESTM E84 requirements. If both the supply duct and the return duct go directly to grilles/diffusers then you are in the clear and there are no special requirements.
Much to my surprise, the Plastic Pipe & Fittings Assoc. (PPFA) have published information that BOTH PVC and ABS meet plenum rating requirements:
http://www.ppfahome.org/pvc/faqpvc.html
http://www.ppfahome.org/abs/faqabs.html
BUT, Charlotte Pipe (and every other resource i've ever seen on the subject) says that PVC and CPVC do NOT meet the requirements for plenum rating:
http://www.charlottepipe.com/Default.aspx?Page=ABSPVCDWV&type=ABSPVCDWV
I've been designing plumbing an HVAC systems for over 10 years, and I have avoided the use of any sort of plastic piping in plenum systems, but moreover, I avoid using open return plenum systems when ever possible - they have way more drawbacks than benefits, IMHO.
It's probably more info than you needed, but it's important to avoid getting turned down by a code official!