It's been happening since a long time of course, but now it's a really great amount hitting the fans. I'll just link to sources one by one to add up some picture.
1. The NY Times reports that already back before Pakistani PM Yousaf Raza Gilani's visit to the U.S., CIA deputy director Stephen R Kappes went to Pakistan and outlined what the CIA knew at the time of connections between Pakistan's security sector and Islamist militants of all kinds, with special regard to the Haqqani Network.
2. The NY Times reports that U.S. government officials now say U.S. intel agencies have proof there was cooperation between ISI agents and operatives involved in the Indian Embassy blast in Kabul on July 7 - these were operatives most likely from the Haqqani network. The evidence the U.S. has is from before the attack on the Indian Embassy (intercepted communications) - however, it wasn't sufficiently specific to stop the attack from happening, although the U.S. had contacted Pakistan over what it did know; that just didn't help.
3. The CIA's increasing and now open pressure on Pakistan's key intelligence service, ISI, comes in the wake of Raza Gilani's attempt to gain more direct control over ISI (and the suspected Agency within the agency), traditionally under the leadership of the Army, staffed by members of the armed forces. Read Indian national security sage B. Raman's account of ISI's history and see how at some points even formally it wasn't sure who ISI was supposed be responsible to, or when its internal policing division was created.
4. Meanwhile, al-Qaida's military commander in the Afghan theatre, Mustafa Abu-al-Yazid, gives an interview to a Pakistani TV journalist on July 21. Jamestown's Michael Scheuer notes how confident al-Yazid must have felt about his operational security if he was ready to invite a journalist to his lair somewhere around Khost, in Afghanistan. Quote: "Ahmed (the journalist who interviewed al-Yazid - P.M.) was greeted not just by al-Yazid and his bodyguards, but also by “dozens of his Arab colleagues.”"
There is a veritable surge of global jihadists coming to Pakistan and Afghanistan now. Excerpt from the Jamestown article:
"Abu-al-Yazid, for example, said in his interview with GEO-TV that the suicide bomber that attacked the Danish embassy (in Islamabad, on June 2 - P.M.) had arrived from Saudi Arabia, though Saudi officials said the man was not an Arab and was not a Saudi citizen (AKI, July 23). The Pakistani media has reported that Islamabad believes there are now 8,000 foreign fighters in the Federally Administered Tribal Areas of northwest Pakistan (The News, July 21)."
5. The Washington Post itself reported in the previous days that AQI's top leaders may have now left the Iraqi theatre for Afghanistan. Foreign fighter infiltration level there is down from 110 per month in summer 2007 to about 20 per month currently, according to a U.S. source. The difference may nowadays be redirected towards Pakistan and the Afghan theatre.
6. One shouldn't exclude that there are other possibilities to enter Afghanistan, besides travelling across Pakistan. But this is a little too much coming up: excerpt from an article by Hamid Mir, a journalist who, according to his own words, can "press" Rehman Malik to concede that there arent's so many foreign fighters in Pakistan (Rehman Malik is PPP co-chief Asif Zardari's confidant who has just been given the role of overseeing ISI and IB, reportedly not liked by Army tob brass as he previously had only been a lower ranking police commander):
"A few years ago, Pakistan was the safest route for foreign fighters to enter into Afghanistan but now they rarely use this old route. Most of them come as tourists and traders directly from Dushanbe, Baku, Istanbul, Dubai, Sharjah, Delhi and Frankfurt to Kabul by different airlines. Many Afghans in Kabul, Karachi, Dubai and Delhi are working for them as travel agents. It is also very easy to make a new Afghan passport for them in Kabul."
This is of course way far-fetched. The safest is to arrive to Pakistan as a tourist, not to Afghanistan, and to then travel to the tribal areas. One can imagine that militants may slip through in Kabul sometimes, but this article is written so that it might as well be an article written to ease the pressure on Pakistan.
7. Meanwhile, India sees stepped up activity by militants also. B. Raman attributes this at least partly to local, Indian cells assisted originally by Bangladeshi AFG veterans of the Harakat-ul-Jihad-al-Islami (HUJI-B) and sees it as one chapter in the "Indianisation" of the anti-India jihad.
8. Meanwhile a bycicle bomb in front of Pakistan's Herat consulate wounds two people so Pakistan summons the Afghan ambassador to its Foreign Office.