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."

Capping was announced on BBC Local News this morning as starting at the end
of this month. Still no info on the TfL sites about it though.