SiteMesh :: decorators and composites for site integration

Integration of pages from multiple sites is neither easy nor pretty. As the SiteMesh docs explain:

There are many web application platforms that are in common use, and even more frameworks. Typically, a web-application is built for a specific framework on a specific platform (e.g. using custom JSP tag-libraries on the J2EE platform, a custom Perl collaboration API using CGI, or a pre-written application using PHP). A web-site may usually consist of many web- applications built with a variety of technologies.

The SiteMesh Overview provides a clear visual example of their approach and solution.

Sounds great, looks like I might want to use it, but what are the Ajax issues?

I ran into SiteMesh because of its use on the Grails project, which is a Rails-inspired framework for Java/Groovy.