Comment: Spring, Scala/Akka/Play (Score 3, Interesting) 390
I have two suggestions that are close to staying with Java:
(1) Check out Spring (http://www.springsource.org/); Spring has a bunch of goodies that make developing web apps easier, and the guys from spring (Adrian Colyer, Richard MacDougall) are thinking really hard about scalable web services. This is a foundation that will let you write in Java but still be prepared for the future.
(2) Even better, don't go with Java, but leverage some of what you learned and pick up Scala. See http://www.scala-lang.org/, or pick up Martin Odersky's book. Think of Scala as what Java would be if someone who appreciated terse, expressive syntax and great convention redesigned Java. Odersky wrote a reference JVM implementation while at Sun, and Scala compiles into Java bytecode and can directly use Scala libraries. (My first Scala project, for example, I used unboundid's LDAP libs directly in my Scala code.) Odersky along with some other luminaries (Viktor Klang, Paul Phillips, etc) have formed Typesafe, and are producing Scala the language + Akka (an actor framework) + Play (a web framework). Outside of play, many people are huge fans of Lift, and it does have some magic that no other framework has.
Remember how you said "modern" web application? Well, Scala supports functional programming, and you can fix functional and imperative code in the same application, which means you can support massively scalable sites by writing clean, idempotent code where needed.
If all this sounds bad, then I'd recommend Django+Python, as it is, imo, the best way for a relative web novice to produce decent code, and the amount you can do with a few hours reading docs and then digging in is shocking.