emal40,
Your math is incorrect.
The Enterprise Value of the company stays constant, not the Market Cap
(EV= Market Cap + Debt)
In your example the company Had MCap of $100m and lets say Debt of $100m. So EV is $200m)
To do the buyback debt goes up and market cap goes down.
Where this benefits Qantas is more in signalling it is undervalued -- expects to return more in dividends in the future from those shares versus return from investing in aircraft/other business
And more importantly puts paid to any hedge funds who had shorted Qantas hoping for an equity raise... those guys will be screaming blue murder.
The buyback is being funded from the Startrack sale/Boeing payment, not from the FY12 loss