The older style £20 note (with Sir Edward Elgar on the back) is being withdrawn.
From the first of July 2010, banks and building societies do not have to accept the old note, but most will, for a few months. The Bank of England will always accept its old notes.
So get under the bed, drag out that old suitcase containing your life savings, and get down to the bank, pronto!
Oh yes, and watch out for those cheeky learners trying to fob off their old notes on you for their lessons.
Full details here:
Bank of England website