Taken from PSU.com : http://www.psu.com/Metal-Gear-Online-beta-delayed-in-NA-+-EU-(UPDATED)-News--a0003457-p0.php

"Konami Digital Entertainment announced today that it has delayed the Metal Gear Online beta test in North America due to unprecedented demand.

Speaking in a statement on its official website today, the company stated:

“We would like to thank all players for their keen interest in taking part in the beta test. However, we regret to inform you that due to extraordinary levels of traffic from all around the globe, we have been unable to run the service to the level we expected.

As a temporary measure, we will be deferring the start of the US public beta test until we are able to stabilize our server.We would like to apologize for all inconveniences as well as confusion caused.

We would like to add that due to the above reasons, we will be making the following changes to the schedule:

1) Starting date: TBA. We will be making another announcement on 22/4/2008 23:00(PDT)
2) Beta Test closure: This will now be on 11/05/2008 at 24:00(PDT) rather than 06/05/2008 at 24:00(PDT)

IMPORTANT: Please note that your voucher code will only be valid up until 05/06/2008, so please make sure to register them by then.”

The full version of Metal Gear Online will be included in the forthcoming June 12 release of Metal Gear Solid 4: Guns of the Patriots. Servers for the European MGO beta test go live today.

Stay tuned. 

Source

UPDATE:

The beta test has been post-poned for Europe as well. The message on the UK website reads:

1) Suspend beta test: As of 21/04/2008 08:00(CEST)
2) Restart date: TBA. We will be making another announcement on 22/04/2008 08:00(CEST)
3) Beta Test closure: This will now be on 11/05/2008 at 24:00(CEST) rather than 06/05/2008 at 24:00(CEST)

UPDATE 2:

The Konami website has been updated and now tells us that the beta will begin on April 25 instead. Here are the US and UK news posts:

US:

1) Start date: Planned for 04/25/2008 24:00(PDT). While the start is planned for the weekend, it may change due to unforseen circumstances.
3) Beta Test closure: This will now be on 05/11/2008 at 24:00(PDT) rather than 05/05/2008 at 24:00(PDT)

EU:

1) Suspend beta test: As of 25/04/2008 08:00(CEST)
2) Planned for 04/25/2008 24:00(PDT). While the start is planned for the weekend, it may change due to unforseen circumstances.
3) Beta Test closure: This will now be on 11/05/2008 at 24:00(CEST) rather than 06/05/2008 at 24:00(CEST) "