The only alternate route is the Bolan Pass further south, between Kandahar and Quetta, which crosses the same mountain range, is more dangerous, and is only open at this time for locals and aid workers.
The Pathans have twice '''defeated the greatest armies of their day'''. When '''Alexander the Great''' wanted to cross the pass, he could not manage it for several weeks, until he bribed one of the local chieftains into assisting him against the ones who were blocking him. At '''the height of British power''' in Queen Victoria's reign, the Khyber was the border of the Raj. Britain fought several wars against Pathans and never completely subdued the area. In the first Afghan war, a force of 16,000 went in and one man came out alive. Pathans were also recruited into the British military, where they were excellent soldiers.
Since 1980, Pathans have been fighting Russians, various other Afghans, American and allied forces, the Pakistani army....and each other.
The Pathans, live (60% in Pakistan, 40% in Afghanistan) and which the Khyber crosses, provided most of the adherents of Taliban. Many are still (mid 2008) fiercely resisting various efforts by US and allied forces and/or the Pakistani government to control their area. If Osama bin Laden is alive, this area is where he is most likely hiding.
Crossing the Khyber has always been something of an adventure. Today it would be far too adventurous for most travelers.
At the top of the pass is the town of Landi Kotal.
From Peshawar to Torkham (the border town) you are required to obtain a permit and travel with an armed guard.
Taxis and buses are available on both sides of the border. See the Afghanistan and Pakistan pages for detailed info on crossing the pass.
The pass is on the Istanbul to New Delhi over land itinerary, though the current recommended route avoids it.