I'd have to agree with mschrag on this one. JSP with Javabeans can be a totally independant layer on whatever database you choose, while Oracle portlets will probably be tied just to Oracle databases. Also compare the number of Java developers with Oracle specific developers and let them know if you were to leave there would be tons of people available to modify your code. Also, if you ever wanted to sell the technology or farm it out to a contracting agency it would be easier to do in Java than portlets.

I'm sure if you found a way to prove your case in a monetary way it would also help. That's what I do at my workplace and it makes a powerful point. "If we buy this Rational software, it will cost us XXXXXX dollars, while if we were to use CVS and Bugzilla it costs us nothing"