Hi Peter
I have learned over the last 11 years working with Veritas/Symantec products that O/S support is only dropped if the hardware vendor has dropped support for the particular O/S.
The 6.5 compatibility guide containted the following warning:
Next major release following NetBackup 6.x will not support Master or Media servers. Back level support of NetBackup 6.x clients will be available until NetBackup 6.x reaches end of support.
We have pointed out this fact to our Tru64 customers when 6.5 was released - one customer has already migrated most Oracle databases to Linux servers and another customer to HP-UX and AIX.
Use this 'show-stopper' as motivation to choose a new O/S... besides, we have learned that in the IT industry 'the only constant is change'.