To be honest, I didn't know we HAD certified Debian 5 yet, but a quick check of the O/S CLs shows that it's there, supported as a client from 6.5.4 (32 or 64 bit) and 7.0 (64 bit only).
NetBackup (tm) 6.x Operating System Compatibility List (Updated January 29 2010) (page 11):
http://support.veritas.com/docs/325328
NetBackup (tm) 7 Operating System Compatibility List (Updated February 17 2010) (also page 11):
http://support.veritas.com/docs/337048
I do see we still reference the old doc even in the 7.x CL, so maybe we SHOULD work on editing it...good catch!