Here is a copy of what it said on mess.be about it being public, public means for everyone:
Confusion over whether the Windows Live Messenger Beta is really public or not! Short answer: YES, IT IS! So stop arguing.
At the start of the week anyone was able to just download the Beta in the language of his or her choice via the links we published. Then, I think it was about half a day later, the freeloading was put to an end and uninvited passports were blocked again (as reported by Messers poodoo*, King Antonius, The_Sickness_14 & Marko). Some people believe it was a temporary server glitch that accidentally let the door open and got fixed as soon as our news spread, but we think Microsoft just added an extra step to their public beta enrollment, to be on the safe side. See, you can just as easily download and use the Windows Live Messenger without being invited, provided that you agree with the Beta's Terms of Agreement first.
1. Sign into your Passport with the account you want enabled for the Windows Live Messenger Beta (eg. by logging into Hotmail). TIP: Use Internet Explorer. Firefox won't work.
2. Click here and click "I agree".
3. Click the "Download now" button on the next page or download the Beta in another language.
The Messenger team has stated that they may reintroduce the waitlist in certain countries if people start signing up at some speed racer pace.