According to AP Pakistan nabbed a few people thought by  some to be related to the attacks in Mumbai:

Security forces overran a militant camp on the outskirts of Pakistani Kashmir's main city and seized an alleged mastermind of the attacks that shook India's financial capital last month, two officials said Monday.
…
Backed by a helicopter, the troops grabbed Zaki-ur-Rehman Lakhvi among at least 12 people taken Sunday in the raid on the riverbank camp run by the banned group Laskhar-e-Taiba in Pakistani Kashmir, the officials said.

AFP reports this a bit different:

The 15 arrested in Pakistan-controlled Kashmir were from an Islamic charity closely linked to the Lashkar-e-Taiba group, which India accuses of being behind the 60-hour siege, the intelligence official said.

"Security forces raided a relief camp set up by Jamaat-ud-Dawa," he said.

The U.S. put Zaki-ur-Rehman Lakhvi on its Treasury terrorist list in May this year. An old post on a Punjabi message board has this bit on Zaki-ur-Rehman:

Writing for Associated Press (May 30, 1999) from Muzafarrabad Mr.
Amir Mirza reported that "… in the mountains that divide Kashmir between
India and Pakistan, militants are training at dozens of camps on Pakistani
territory." He, along with other journalists, interviewed Zaki-ur Rehman
Lakhvi, chief of Lashkar-e-Tayyaba, one of the most militant fighting groups.
In this interview Mr. Lakhvi claimed that "there is no shortage of recruits."

The man is obviously not unknown. But is he guilty in this case, or just a convinient target?

There were rumors on Saturday, later denied, that Sec.State Rice had given an ultimatum to the Pakistani government do something within 48 hours. Now Pakistan has done something. Whether the people nabbed now are really related to the Mumbai attack is an open question.

And what will be the next step?

It is doubtful that the Pakistani government can and will simply send
off the captured folks to India. There are legal reasons against this
as no extradition treaty exists between the countries. The internal
political situation will also not allow it, as Zaki-ur Rehman is to many Pakistani not a terrorist, but a hero who fought for the freedom of Muslims in
Kashmir.

Pakistan could put the nabbed people on trial. But it may have no evidence
against them except what Indian 'sources' leaked to Indian media. An then what?

Some Indian TV channel is speculating about military action against Pakistan.

B. Raman, hawk and former chief of India's foreign intelligence
service Research and Analysis Wing, says that is the wrong stuff to do.
Instead he is urging India to copy the U.S. and to not care about
international law.

Why would India need to show evidence that Pakistan was behind the attack, he asks:

What
evidence did they have before Bill Clinton ordered the Cruise missile
attacks on jihadi training camps in Afghan territory in August,1998?

What evidence did they have against Al Qaeda and the Taliban before they bombed Afghanistan from October 7,2001?

What evidence did they have against the Saddam Hussain Government before they invaded and occupied Iraq in 2003?

In every case affecting American nationals and interests, they
bombed and then collected evidence. They did not wait till they had
collected all the evidence possible before they bombed.

Raman wants the Indian government to reactivate the operational arm of its
foreign intelligence service and to get active within Pakistan. Follow the U.S.:
Just spread terror in the land of the alleged and perceived enemy.

The objective of the action should be to force Pakistan
to act effectively against the LET and its terrorist infrastructure. It
should also be to mount a no-holds barred covert operation against the
LET through our own resources and methods.

…
A divided Pakistan, a bleeding Pakistan, a Pakistan ever on the
verge of collapse without actually collapsing—-that should be our
objective till it stops using terrorism against India.

A divided and bleeding Pakistan is of course what Pakistan is already
today. Creating more strife in Pakistan would only create more
terrorism spreading from Pakistan into India.

Raman knows this:

We should be realistic enough
to anticipate that Pakistan will step up terrorism in Indian territory
if we adopt such a policy. This should not deter us from embarking on
this policy. The policy of active defence against Pakistan should be
accompanied by time-bound action to strengthen our counter-terrorism
capability at home.

So terrorism from Pakistan in India should not deter it from raising terrorism in Pakistan. Why then
does Raman believe that such action by India can deterre Pakistan?

There is some very faulty and dangerous logic behind such thoughts.

That may not prevent their implentation.