No. There is no evidence of hackers attacking the grid. The only news regarding that hyppothesis appeared only after the CBS article.
By the way, yesterday (oct-10) around 10:30pm there was a new blackout affecting the brazilian south western states.
Again the same stations from Furnas (the electricity transmission company for the region) were affected. These stations use capacitor banks from ABB and are (were?) the only of this kind in the world for 735kV.
The blackout in 2005 was a human failure. One transmission line went down, the team recovering that line made a mistake and instead of activating the repaired line disabled the backup line. Result: 3 states withou electric power.
The blackout in 2007 was due a circuit breaker shutting down one line, the same happening after in the backup line, that could manage the excess load (this happened during peak hours, 5 p.m. during a working day).
Ok, these are official explanations and the blackouts may have been caused by evil hackers but, in this case, the brazilian government made an excelent job holding that information for years, leaking now thanks to an american former military that may have some vested interest spreading fear.
One good reason why computers can do more work than people is that they never have to stop and answer the phone.