Hmm...an interesting subject.
I think the discussion so far has neglected (unless I've missed it, which is quite possible) the effect of location. Minas Ithil is basically in Mordor, and the pass that it guards is rather narrow, giving those who sieze it a distinct advantage. Taking this into consideration, I think the term siege fits in quite well. The Nazgul, and whatever army they may have assembled over the course of time, could easily hold such a pass from any returning force of Gondor's while they are sieging it.
A secondary idea to this is that the siege was not on-going in the sense of a constant pounding. With Mordor being 'desolate', I think that the siege could very well be off and on as the Nazgul and their army can manage it. Even if Gondor's army is in short supply of available troops, any force of them is enough to at least deal with orcs. So while Gondor may send small replenishing units of soldiers to garrison in Ithil, the length of the siege, and a potential off and on again nature would in time subdue it.
Yet another side to the location idea is that Ithil is not in Gondor proper. Historically, occupying armies have a very hard time holding a place their people may not care for, considering when it is outside of the homeland. What is there for Ithil to offer, besides as a bastion of defense? And with Sauron himself gone for so long (or at least not on Gondor's doorstep), there may not be enough concern to warrant funneling soldiers to Minas Ithil.
On the question raised by food supply: Barad-dur held out in a siege for seven years, if I recall correctly. Orcs need food too. So then, if a seven year siege can be enacted and maintained, and the defenders can resist it well enough, why not a two year siege at Minas Ithil? It would certainly have fewer mouths to feed.
Anyway, I'm probably ranting and may have gotten off-topic. So I shall retire.
