Viber has been on the way for a long time... it probably won't get to the Windows Phone platform till Windows 8.
And there are rumours going around that the current phones won't be receiving the Apollo(Windows Phone 8) upgrade.
I wouldn't bother with the rumours, at the end of the day their just rumour and there's a mixed bag of them.
On a personal opinion if it was to come to 1st gen and 2nd gen devices not being able to fully support Apollo due to hardware requirements Microsoft will still deliver an update to suit the hardware and add features. They cant afford not to, cause it will piss of nearly every single windows phone fan and all the early adopters .
If you follow the trend of Windows (excluding vista) and Windows Phone, each OS requires less and less resources to run. In my opinion this rumour that we might not get it is a full bag of air.