Can I use a java servlet to check if a browser is using 128 bit encryption?

[I know that for Netscape, the user agent in the request header has a "U;" that indicate that the browser is using 128 bit encryption. What about IE?]

Unfortunately there is no standard way to get that piece of information. The one you've mentioned, as you probably know, it's not a standard solution and there is no way to guarantee that it will be maintained in the future.