[Inziladun]

Originally Posted by Alfirin

1. Maybe Denethor didn't start using the Palaintir until after Boromir left. In fact his long abscence, and possibly knowedge of his death (via Faramirs vision and the split horn) may have been what drove Denethor to start using the Palantir in the first place.

Gandalf seemed to believe he had been using it before then.

But his wisdom failed; and I fear that as the peril of his realm grew he looked in the Stone and was deceived: far too often, I guess, since Boromir departed.

'Far too often' suggests he had made use of it before, though he had been a bit more careful before Boromir's departure.

Originally Posted by Alfirin

2. Rivendell may have been out of the Palantir's range. I seem to recall something in the Esaay of the Palantiri in the UT that said that the distance a palantir could see was related to its size, maybe the anor stone was too small to have that range. A clue for this may be in the fact that the two largest stones (Osgiliath's and Amon Sul's) were placed in positions where they could have been used as relay stones (one in Gondor one in Arnor) for messages coming in from the other stones.

I think the quote from UT stated that the images seen in the minor Stones would be small. It seems the larger ones were best suited for long distance communication, but I wouldn't think the distance between Minas Tirith and Rivendell would have been too far for Denethor to survey, though it probably would have been extremely taxing on his strength.
However, in order to look on Rivendell, wouldn't Denethor have to possess already knowledge of its location or appearance? If not, why couldn't Sauron have used the Ithil Stone to find the Shire?

Originally Posted by Alfirin

3. Whatever magic Elrond had that made Rivendell so hard to find may also have prevented it's being scryed by a Palantir. If it didn't then sauron himself could and would have immediately known EXACTLY where Rivendell and Lothlorien were.

That's an interesting idea. The Tale of Years tells us that Rivendell was founded by Elrond in the year 1697 of the Second Age. 3319 was the year of the Downfall of Númenor, and the time at which Elendil and sons arrived in ME, bringing with them the Palantíri. Elrond, I suppose, could have made his refuge 'unplottable' ( to use a Harry Potterism ), but he would not have done so as a conscious defense against the Palantíri, as he likely did not even know of the Stones when he founded Rivendell. If he was aware of them, they were in the hands of the Faithful in Númenor then, so why would he have felt the need to block them?

edit- x'd with Boro