[Legate of Amon Lanc]

I am not going to join with any long discussion regarding this question, I believe the only and best evidence we have is what has been already mentioned, and I side with Inzil. Eru seems to abide by the laws of the world, or even more, he actually seems to let the world abide by the laws set by Valar in their stewardship (such as the Ban on Noldor). At the moments when the laws of the world need to be drastically altered, such as destruction of Númenor (and I disagree with jallanite's interpretation, but whichever the case, it still is undeniably such a big change that Eru needed to be consulted) and creating the Straight Path, Valar ask Eru and he responds. The second alteration of that calibre that I know of is, with a questionmark, the sending of Istari (there is a note in the "Istari" essay in the Unfinished Tales that Manwë *perhaps* asked Eru for permission to send the Istari). At that point (in the Third Age), the Valar already are also more on the side of Eru on the scale of intervention with the world: they basically don't intervene at all. I would phrase it the way that during Third Age, one of the "natural laws" is also that Valar are not anymore directly intervening with the world. I would argue that it actually becomes exactly such an unchangeable law just like the fact that things fall down when dropped and so on.

Eru actually, from the beginning, is a rather unintervening creator - a deist creator, if you will. He lays out some theme in the beginning (perhaps we could say, basic set of rules in which things may function, i.e. the basic unchangeable laws? Some mathematical types would certainly say that, such as if G. Leibniz got Silmarillion into his hands and was supposed to write his Theodicy based on it in regards to Eru), but the Valar are the ones who create based on it. He gives life to it, and as we see with Aulë's Dwarves, he sustains the life (we could presume also that he sustains the world). But he does not really do anything else, except for the few dramatic occassions where really the highest power is necessary to intervene, such as the case of Númenor.

But Eru does not change the laws of the world, normally. It says nothing about whether he could or not, though. So it is not about the potential to do something. Tolkien's tale does not speak, at all, about Eru's potential. We can only conclude based on

If I were to answer this, I would copy Inzil: Based on all evidence, Eru probably is (meant to be) omnipotent, but he actually seems to have decided not to intervene with his creation very much. That is also one of the "natural laws" of Arda. Obviously, since it is set by the creator himself. If he breaks these laws, it's a very rare occassion (I really think Mr. Leibniz would have been even happier with Silmarillion instead of the Bible, since the discussion of the creator's potential is far more straightforward in the former).

Speaking of which brings me to one final remark. As much tempting as it is, let's be careful not to confuse Eru with (Judeo-Christian) God, since even though the parallel is obvious, you cannot obviously put an equation there, and already such mistaking of terms can lead to different conclusions. (Actually, I slightly suspect the author of the first post of such "confusion in terms", but I may be mistaken and it's merely an inquiry applying the famous "god's potential" question to Eru, which would be actually good question; I am however very much aware of the confusion of terms in jallanite's posts, so let's be careful about it.) I think the similarities show better on a deep level on some specific occassions, but the general picture remains very different: overall, the Biblical God is defined by intervening, and acting in very personal manner, Eru in general shows these aspects only rarely and is closer to the idea deistic of god who creates, sets the rules, winds up the clock and then leaves his creation alone. (Of course, that is disproved on closer look by realising the fact that he probably acts continually to sustain the life of all creatures, such as in the abovementioned tale of the Dwarves. But again it is questionable whether Eru sets "rules for life" and lets it go - the basic law being that from now on every Dwarf born, as much as human, is a living being from start - or if he needs to "renew" this life with every new creature born, "breathe life" into every new generation again or else the Dwarves would again fall into nothing, or if he even - as some theologies maintain about the Judeo-Christian God - continually "is present", to keep the world sustained, keep the living creatures breathing every single moment. From superficial reading of the Silmarillion, though, I think it seems even as if Eru didn't do even the former, just created the world in the beginning and that's it.)