I registered even before december last year and didnt get a beta invite yet... so probably its still too early to get really excited, as there are thousands of people that registered later... especially over the last time.
Makes only sense of course if they send out the codes in the order as people registered.
I have to correct myself. i never got an invite email from viber, but using the email i used back then to subscribe i could register here http://www.viber.com/android-beta/
without any problems. i just had to try some email addresses as i didnt know anymore which one i used back then. I didnt recive a sms but the activation call worked without any problems.
So yeah, even if you didnt get an invitation mail it might work. And YES, i checked my spamfolder before!