THREAD RESURRECTION - you can use a fermemeter on plastic. Compared to glass the only real diff. is that it prolly takes a little bit longer to display change in temperature, like when fermentation is first starting and the wort starts heating up. But this delay isn't significant enough to warrant not using them on plastic. And it will show you the temp of your wort well enough for fermenting purposes.
The biggest issue with this is when the ambient temp is drastically diff. than the wort temp. I usually don't trust them if a change in temp is suspected, like taking a lager out for a D-rest. I wouldn't trust the temp. on the fermometer until it has equilibrated for a couple hours.