eldarerathis is right. The PowerPlate is a two part system. 1. The Base and 2. The Receiver. The receiver clips onto the phone and is specific to the phone. It looks like the powerplate system replaces the battery cover on the blackberry and is a snap-on cover for the iphone.
The product description on Brookstone's site (and the Amazon one mentions this, too) seems to indicate that you can't. They specifically list that there are receivers for iPhone 3G/3GS and Blackberry Curve 8900. You can't charge something without the receiver/case, so if they don't make one for the Eris (which it appears they do not) then it won't work.
That being said, you could always stop by or call your local Brookstone and ask, I suppose.