On Tue, 13 May 2008 13:42:40 -0700 (PDT), Paul Oter wrote:

What alternative exists for topping up
the cards besides carrying extra 10- or 20-pound notes?

You could try topping up your Oyster with a credit card at the ticket
office at any tube station (i.e. by handing your card over to a human
rather than sticking it into an unattended machine). If you top up
with cash you'll also need to hand it to a human at the ticket office.

You can top up with cash at any ticket machine (with coins as well as
notes).

Also, if I have unused cash stored on my Oyster Card when I finally
depart back to California, do I lose it?

No, you can get a refund at any tube ticket office.

This used to be limited to £5 - is that still the case?