I tested your sample app in an old G2.1.5-snapshot with tomcat 6.0.18-G678601, it works very well.
And I also just tested it in the newest G 2.1.5-snapshot, it seems works without any problems in my environment.
I have a sample application which uses the Web messaging service to simulate stock quotes to a Dojo-enabled client application and i use Tomcat NIO connector for comet support. I tried deploying the application in Geronimo 2.1.5-Snapshot server, the stock quotes update doesn't happen properly but it works well in Geronimo 2.1.4. A look at the requests in the firebug shows that the bayeux connection always terminates in a very short time interval. however, I don't see any exceptions or errors in the console.
My investigation suggests me that Geronimo 2.1.4 uses tomcat version 6.0.18 while Geronimo 2.1.5 uses tomcat version 6.0.20 and i also see that there is difference in the packaging of tomcat too. Any suggestions, pointers on this will be very helpful. Please help.