Hmmm, I agree with the conclusion that Tolkien implied that Celebrian was raped by the Orcs. Lets see if we can come to a viable hypothesis by asking ourselves several questions:
1. How do Orcs procreate? This question is of importance because the question whether Celebrian was raped depends on the fact whether Orcs had sexual feelings. If Orcs reproduced asexually and had no sexual feelings at all, than why would they rape Celebrian? Indeed, if they were asexual then the very notion of rape must have seemed alien to them, since the act of fornication would be completely unknown to them (including the intimacy of it) and they probably would've been unable to conceive how much being raped would have damaged Celebrian, both physically and psychologically (and I'm not even talking about the Orcs lacking the *ahem* 'proper equipment'. However, it seems it's pretty clear that Orcs reproduced sexually. They clearly have families (Bolg, son of Azog) and the Silmarillion clearly states that Orcs reproduced in the manner of the Children of Iluvatar. It is therefore obvious that Orcs reproduce by having sex.
2. Did the Orcs have the opportunity to rape Celebrian and is it likely that they would have done so? It is strange that the Orcs abducted Celebrian instead of killing her outright, which would seem to be an Orcish thing to do. Celebrian would have had an escort but she's the only elf who was taken captive. This means that the Orcs probably had 'special plans' for her in store. One must wonder why the Orcs would have spared her while killing the others. It is unknown what the make-up of her escort was but one would assume that she has some ladies-in-waiting with her. These appear to have been killed, though it could be that I'm speculating too much here. Celebrian was the wife of a high-ranking elflord, so this would add to her value, but I'm not sure whether Orcs hold peopel for ransom. When they are not under the influence of a Dark Lord Orcs seem to make a living with raiding (as seen in the Hobbit where the Goblins intend to raid villages with the Wargs) and brigandeering. From what I can tell they generally tend to have a 'No prisoners, no survivors' attitude towards their victims. Tolkien has also made no mention of any ransom demands from the Orcs, so I feel that the 'ransom' scenario is unlikely.
This leaves the 'sadist' scenario. The Orcs took Celebrian to their dens to have some 'fun' with her before killing her. Their entertainment was cut short by Elladan and Elrohir's rescue attempt (I assume this was done with a strong force of Elves under their command). Tolkien states that Celebrian was tormented and that her psychological wounds were so severe she could no longer dwell in Middle-Earth. How likely is it that rape was part of that torment? Orcs are sexual creatures and pretty evil creatures at that. It seems unlikely that Orcs would have had any sort of aversion to raping captured women. We also know that Orcs love to defile and destroy beautiful things. To an Orc, what could be greater than to defile a beautiful elflady? What is the most degrading, the most defiling thing that one can do to woman? I'm afraid the answer is pretty obvious on that one.
This would also explain why only Celebrian wasn't killed outright. Her high status probably enticed the Orcs to draw out her defilement as long as possible, while they quickly killed off her escort of soldiers and lower-ranking ladies.
Conclusion: While the text itself is rather vague on the subject, it would appear to me from a logical POV, taking into account the situation and especially the nature of Orcs, that it is quite likely that one of the torments Celebrian was subjected to was indeed rape.
I hope this post wasn't to vague or fuzzy, I tend to lose track when writing large posts like this.
EDIT: Edited for typos n such.
