Andy,
In general, IPMS "encourages" the use of plastic as the primary construction material in most modeling genres. However, as an organization, they also realize that the aftermarket and alternative materials industries have really taken off during the past decade or so. Additionally, in certain genres (ie, Figures) the vast majority of material used is resin. IPMS has always attempted to embrace as many of these alternate materials as possible.
Now for the disclaimers:
One of the biggest problems IPMS Contest Directors saw during the 1990's was the influx of bogus modelers entering contests with pre-manufactured diecast automobiles. The IPMS rules have always stated that construction of the model has to be the "sole work of the modeler" (exception: Group Entries). Therefore, there is an ongoing dialogue within the IPMS community regarding the use of "non-plastic media" and what constitutes an eligible model.
There is some variation in what is allowed from contest to contest. The IPMS National Contest rules (Competition Handbook) are not mandatory for local contests and you will see both more liberal and more stingent rules. The best thing to do is check with the local Head Judge before the contest to get a ruling on your particular model. The same holds true if you want to enter a Regional or National competition.