This BBC report summarizes conflicting accounts of an incident on the disputed border between Pakistan and Afghanistan. This is a reason to have a national security debate as scheduled tomorrow night, or as soon as possible. Pakistan is entitled to respect, and has a right to defend its borders, but Afghanistan also has a right to pursue insurgents across the border, with or without American help. Pakistan also has as much responsibility as any nation to deal with international terrorists within its own borders. If it can be proved that bin Laden and other al-Qaeda leaders are there, the government should not be protecting them.
There's a line of argument that suggests that Afghanistan should be conceded to a Pakistani sphere of influence. You can find that viewpoint expressed here (as the author debunks "Myth 3"). I don't care for this argument because it seems to take Pakistan's side unconditionally in its disputes with India. Pakistan has supposedly always wanted a compliant government in Afghanistan so that its army can maneuver there in the event of a war with India. I see no reason to give Pakistan an advantage in any such conflict. Nor do I think that Afghanistan thus automatically becomes an ally or client state of India. The country ought to be capable of neutrality, or at least should have the opportunity to choose its allies for its own reasons.
On the other hand, there is an ethnic affinity along the border region among the Pashtun tribes. While this suggests a natural friendliness between Afghanistan and Pakistan, it should be remembered that Pashtuns are not the only ethnic group in Afghanistan, and that many others feel no affinity for Pakistan at all, even though Pakistanis are fellow Muslims.
All that being said, maybe it should be up to India to prevent Afghanistan from becoming a Pakistani puppet. To the extent that there is an American interest in the region, it's limited to pursuing and capturing bin Laden, and this really shouldn't require an army in Afghanistan. But I'd like to think that Senator McCain and Senator Obama are better informed on the subject than I am, and I'd like to see them discuss the topic, tomorrow or soon.