WebSphere 6.1 can work with Tapestry ... if configured correctly. WebSphere doesn't quite conform to the Servlet API specification.
This can be addressed by installing fixpack 9:
In addition, you must configure the setting com.ibm.ws.webcontainer.invokeFiltersCompatibility to true. With all of that in place (plus some additional code inside Tapestry that patches around WebSphere's odd behavior), you should be ready to deploy with WebSphere.
TODO: Discuss logging configuration for WebSphere.