Seems confusing. Ok, let's see here

An example
Ex date is 20th August 2011.
Entitlement date is 22nd August 2011.

If you bought the shares on 19th August, and sell it on 20th August, you will still be eligible for the dividend.

You must be wondering, why Entitlement date is 22nd August. Remember, if keep the contract for over day, then the money need to be clear after 3 days, that is why entitlement on 22nd August.

19th (T+0) Latest day you can get to entitle for dividend. Keep it overnight, then your contract will be effective for dividend already, because there will be a statement sent to the registra.

20th (T+1) Ex-date, you can sell already. But need 3 days to clear $$$.

21st (T+2) 2nd day of Ex date. Meaningless

22nd (T+3) Entitlement for dividend for contract that kept overnight before or on 19th Aug to 20th Aug.