Well, the role of artillery has changed through time. It's no longer the mass-casualty weapon of the 18th century, or the rolling-barrage weapon of WW1. It's likely it will change again with this new conflict.
The principal enemy in the War on Terror is the Arab infantryman, armed with assault rifles, high explosive and RPGs, fighting on his own ground. The typical posture of this kind of fighter is to fortify a position and ferociously defend it - the classic example being the Prophet at Medina in the fifth year of the Flight. The events at Najaf and Tora Bora were in much the same pattern. Such fighters are often unconcerned with their own deaths or those of nearby non-combatants. Being surrounded is no problem.
I'd see artillery in this kind of conflict as having two main roles i) controlling the topology of the battlefield, and ii) inderdiction.
By controlling the topology I mean that a commander with modern artillery can decide which buildings stand or fall, which bridges. Given enough firepower he can even remove forests or mountainsides. This kind of capability is vital to capture strong positions.
By interdiction, I mean preventing enemy activities such as reinforcements, checkpoints, retreats or advances. These will also be very important given that the enemy may have more concentrated forces locally. The classic example of this Ia Drang where Hal Moore's 7-Air Cav batallion held off a PAV brigade, primarily with artillery barrages.
My thought is that the classic weapon for this type of activity will be the trusty 105mm gun (air and helicopter movable to the high ground), but that the ammunition may need to be re-shaped to support this type of fighting. One can imagine a localised, very high temperature explosion might be better for precision destruction of buildings, for example.
It's a really interesting question. Sorry about the slightly long reply - the question captured my imagination