Unable to get it to reproduce on Mac OS X at all (FF3 and IE6), with the compression verified using FireBug and the Java debugger.
See notes here: http://www.ruby-forum.com/topic/149617
I suspect there's a misconfigured server between your Tapestry application and your client that is responsible for the garbling.
I'll leave this open a little bit longer, but right now it is headed for "could not reproduce".
Workaround: contribute "application/json" to the ResponseCompressionAnalyzer; this will identify the content as not compressable, which is what you want.