The ASP.NET authentication web service is a really cool way to avoid using the UpdatePanel, but you may want to think about it when you've got a really funky custom-built Membership provider.
When I started building a conventional ASP.NET application, I added in the code to load user information into the membership provider as I was retrieving the user object from the database anyway. It got me a little extra performance and everyone was happy.
I then added an extra call from the client-side script to call another web method that would load the user object into a session variable to avoid errors on other pages.
I guess sometimes, a performance hack can come back to bite you.