UPDATE – I can confirm that upon logging into Bitstamp, a warning is displayed:

DO NOT DEPOSIT TO PREVIOUSLY PROVIDED BITCOIN DEPOSIT ADDRESSES. Deposits sent to previous address will not be honored. New deposit addresses are forthcoming.

Bitstamp has is alleged to have requested that users of the exchange stop sending bitcoin deposits to their Bitstamp accounts after some users began reporting inconsistencies with their balance after transferring bitcoin to the exchange over the past hour. The email that is alleged to have come from Bitstamp reads:

Dear customer,

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 can not be affected.

We will send you more info as soon as possible.

Best regards,

Bitstamp team

As of this time, Bitstamp has yet to confirm the legitimacy of the email via their website, Facebook or twitter accounts despite reports from users said to have received the above message. Given the volatility in the bitcoin price over the past 24-48 hours, it would make sense to remain sceptical until such time that Bitstamp confirms or denies the authenticity of the email. If authentic, this is something which could've been made clear with a simple GPG clearsign message to begin with.