Mumbai Attack: Major Scenarios

1) Apparent official Indian version-- Pakistan terrorist group in concert with ISI perpetrated the attack, because they are flaming radical Islamists and hate Hindus.

2) Possible false-flag attack by Indian group, to be blamed on Pakistan, meant to escalate Indo-Pak tensions and give casus belli for India to attack Pakistan (note, US is down-playing role of Pakistan currently).

3) Deep covert version-- Pakistan terrorist group in concert with ISI perpetrated the attack, which was ultimately run by British intelligence/PTB, with the goal to escalate Indo-Pak tensions and start a war leading to a nuclear exchange.

4) Deep DEEP covert version-- Indian terrorist group in concert with Indian intelligence agencies and Pakistan ISI perpetrated a false-flag attack, an attack which was ultimately run by British intelligence/PTB, with the goal to escalate Indo-Pak tensions and start a war leading to a nuclear exchange.

UPDATE: US intelligence agencies gave India warning on the attack, indicating likely involvement of the US at some level (Pakistani ISI is closely linked with the CIA, so no surprise really):

The Associated Press reported that the Bush administration had warned India before the attacks that terrorists appeared to be plotting a mostly waterborne assault on Mumbai, quoting a senior administration official who spoke on condition of anonymity because of the sensitive nature of intelligence information.

Also, in the same article, clearly, all indications are that Pakistan will be officially blamed for the attack.

UPDATE 2: Some evidence for a False-flag attack here and here.