I've heard that it'll take a couple of hours to put right so that means by 11ish or 12 noon today it will be up and running normally again.
So if you're an early bird and need to get anywhere, if I were you I wouldn't be reading this much longer & would get out and about on the Tube and buses while it lasts!
UPDATE - 11am It's working again but some gates are still open and buses appear to be taking a little longer to get back to normal. Here's the statement from Transport for London:
"Due to a technical problem with the Oyster card computer system, card readers across the network were not accepting cards from around 5.30am until 10.30am on Saturday 12 July.
The problem has been rectified and cards are now being accepted across London Underground, DLR, London Overground and on those First Great Western, First Capital Connect and CTC that accept Oyster. A number of buses may still be affected but all passengers are being accepted to travel.
All passengers who incur a maximum fare on Saturday 12 July will be given an automatic refund on Tuesday. They do not need to take any action.
We are investigating the cause of the problem, will ensure that any necessary refunds are honoured and apologise to our passengers for any inconvenience caused this morning."
UPDATE - Sunday 13th July - Just had an interesting update in the comments as it appears things might not be as rosy as the TfL statement above makes out:
"Anyone who touched in or out yesterday morning has probably had their Oyster card disabled (code 30) and will need to change their Oyster card at a ticket office using the failed Oyster card procedure. If it is a student, child card etc, then they will need to contact Oyster for a replacement. Even our staff have had their passes disabled. Yesterday by 22:30 my station had changed nearly 600 oyster cards. We normally change about 5 a day."
Have fun on Monday!
UPDATE Monday 14th July - Just heard on the radio that 40,000 people tried to use their Oyster cards on Saturday morning and if you were one of them you'll need to get it replaced. So pleased I didn't use mine at all on Saturday.
Here's some of the official word from TfL following from their previous statement
"A number of cards used on London Underground before 9.30am yesterday may not be working as a result of yesterday's technical problem. Customers who topped up their cards at Oyster Ticket Stops during the day may also have been affected. Customers whose cards are not working are advised to go to their nearest London Underground ticket office where they will be able to exchange their card for a replacement.
A small number of Freedom Pass and Young Persons Oyster card holders may also have been affected. They will be required to apply for replacement cards through their relevant local authority and through the young persons travel card helpline respectively. Our staff will be instructed to allow these card holders to continue to travel whilst replacements are issued.
We are investigating the cause of the problem and apologise to our passengers for the inconvenience caused. "