Metal Gear Online beta is already up and ready for downloading onto your PS3. It actually is supposed to be downloadable tomorrow, April 17, though we're not complaining. If you are for some reason really anxious to slap this sucker on your system, then by all means, take advantage of this early gift.
Go on, follow the instructions on your voucher and grab the 741MB beta. The beta of course won't start until April 21, so don't expect your download to do anything other than sit prettily on your XMB.