The Spring issue of Tube magazine dropped through my door this morning,
with the following on page 39:

"If you use an Oyster Pre Pay, you'll get even more value for money
thanks to its new feature - capping. Due to start in March, customers
who make a number of Pre Pay trips ... blah"

Oystercard.com doesn't seem to mention it and the FAQ entry regarding
capping in the support section still says:

"We are looking at ‘capping’ the maximum amount you will pay in any one
day but have no firm details or a planned introduction date at present."

--
Ashley Brown