I caught Peter Brookes from the Heritage Foundation on his Blackberry at Dulles for his quick reaction:

Story is still breaking but it could be a number of groups including Pakistani Kashmiri militants or even a group called the Indian Mujahedeen, an Islamic group based in India. (A group called Deccan Mujahedeen has claimed responsibility…)
The reported targeting of India’s business and entertainment center and foreigners may lean toward an indigenous, separatist group rather than a Kashmiri group.
On the other hand, Pakistan’s president has made some positive overtures toward India recently, meaning that some Kashmiri groups could have undertaken the attack to prevent an improvement in relations between Pakistan and India, fearing that Islamabad may knuckle under to Delhi on the Kashmir issue.
Al-Qaeda is a possibility although they haven’t had a strong presence in India to date, but the precision and sophistication of the attack certainly reminds us of its tactics and techniques.
We’ll be staying tuned…