Bitstamp is one of the world's leading and most famous Bitcoin exchange sites, having been in operation since before the boom of the Bitcoin happened and haven't had any sort of issues with payments - Until now.
The company send out an email to its users, after many discovered that their account balances were in 0, which stated that they are having problems with their hot wallets:
Today our transaction processing server detected problems with our hot wallet and stopped processing withdrawals.
You should STOP SENDING bitcoin deposits to your Bitstamp account IMMEDIATELY as private keys of your deposit address may be lost.
Your bitcoins already deposited with us are stored in a cold wallet and cannot be affected.
We will send you more info as soon as possible.
After having send out the message to their customers, Bitstamp further created a message on their Bitcoin deposit page, which stated that users should not deposit to the previously created Bitcoin addresses, but instead await new addresses to send to. They also stated that any new deposit made to the old address will not be honored in their system.
With the Bitcoin having surged in value and with Mt. Gox fresh in mind, several people took it to Twitter to warn others about the issues. Some claims to having had withdrawal problems for some weeks with Bitstamp, while others claim to not have received replies to support inquiries for more than a week.
Whatever is going on, it seems rather suspicious the way it has been going down. It might simply be due to an integration to a new system, it may have been an attack which compromised wallet details or a third scenario - We won't know until Bitstamp makes a statement.