: An Heir of Isildur doesn't look a certain way. They don't metamorphose physically and alter their appearance. Aragorn had to have shown Sauron what he looked like at some point in time, such that he would be unrecognizable to Sauron in the present day, but still threatening enough to give Sauron reason to believe he had taken up the Ring, or was about to.

: Thorongil matches this requirement.

: : I'm interested in the Thorongil idea. But if there is a
: : question about what "the heir of Isildur" looks like (other
: : than like Aragorn), what does Thorongil look like (other than a
: : much younger Aragorn)?

: He would not only have been younger, he would have been arrayed as a soldier of Gondor, perhaps standing under a Gondorian banner. Remember, the first major assault was directed at Gondor as planned, but it was rushed in part because Aragorn revealed himself to Sauron. If Aragorn showed Sauron an image of Thorongil, captain of Gondor, and implied that he had the Ring (or at least gave Sauron reason to infer he had the Ring), then Sauron had all the more reason to crush Gondor as quickly as possible. He couldn't afford to let a successful captain rouse Gondor from its lethargy.

: : And is it really true that Sauron could only track Aragorn's
: : movements with the palantir if he had already seen him? I don't
: : doubt you, necessarily, but I can't find a passage that says
: : this.

: I'm no more an authority on the Palantiri than anyone else who has UNFINISHED TALES. Denethor was able to see things from far away as they happened, or perhaps to look back in time. Aragorn also used the Palantir to look at events unfolding far away (and hence he learned about the fleets threatening Gondor). Every passage I've looked at indicates that visual imagery was very important. Even Pippin's exchange included visual imagery. Sauron "saw" Pippin, and Pippin saw Sauron.

From The Last Debate:

'The Stones of Seeing do not lie, and not even the Lord of Barad-dur can make them do so. He can, maybe, by his will choose what things shall be seen by weaker minds, or cause them to mistake the meaning of what they see ...'

It may be possible that Aragorn was able to trick Sauron in the same fashion, presenting himself to the Dark Lord in some other guise. It is also possible that what Sauron saw was an 'image' which had been stored in either the Ithil- or Orthanc-stone.

Endnote 18 of the essay The Palantiri discusses the ability of the stones to receive and store images, even when not being actively used:

They [the Palantiri] retained the images received, so that each contained within itself a multiplicity of images and scenes, some from a remote past ... They themselves could be and usually were kept in the dark, because it was much easier then to see the scenes that they presented, and as the centuries passed to limit their "overcrowding". [Double quotes are given by the author]

It is possible (assuming the above to be accurate) that the image which Aragorn presented to Sauron may have been that of Isildur himself, or maybe Elendur or even Elendil. That such a thing is possible is suggested (IMO) by Gandalf's words to Pippin:

'Have I not felt it [the desire to use the Orthanc-stone]? Even now my heart desires to test my will upon it, to see if I could not wrench it from [Sauron] and turn it where I would - to look across the wide seas of water and of time to Tirion the Fair, and perceive the unimaginable hand and mind of Feanor at their work, while both the White Tree and the Golden where in flower!

As far as Sauron's ability to track Aragorn, this would have diminished as Aragorn moved closer to Minas Tirith. As discussed in Unfinished Tales, there was a 'proper distance' at which the Palantiri were most effective. For the lesser stones (Minas Anor, Minas Ithil, Orthanc and Fornost - the stone of Elendil being a unique case), this distance was approximately 500 miles. At greater or lesser distances (to what degree is unspecified), these stones had more difficulty resolving images. It is noted that the stones of Minas Anor and Minas Ithil (prior to its fall) were too close to allow for communications between them.

Still, I agree with Mr. Martinez that Sauron would probably have been able to track Aragorn's movements using the Ithil-stone. However, what he could have done with this information is another matter. His plans were in place; his armies on the move. I doubt there is much Sauron could have done other than to watch as Aragorn rode through southern Gondor, defeated the Corsairs at Pelargir, and sailed up the Anduin in the captured ships. Even if he had sent one of the Nazgul to warn the army beseiging Pelargir of the approaching peril, I doubt they could have withstood Aragorn and his army of the dead.