Entwives

[The Might]

I think there might be something to Aganzir's idea.
It may well be possible that Ents took the shape of the tree that they guarded as for example Quickbeam who is said to have guarded Rowan trees also resembled them.

Due to their many animal-like characteristics I would speculate that the first spirits that became shepherds of the trees belonged to animal kingdom, looked plant-like (probably to blend in with the other trees) and could assume the form of the trees they guarded.
As such I believe breeding would be pretty much like in most other animals.

An interesting quote for this is:

... some (Galadriel) were [of the] opinion that when Yavanna discovered the mercy of Eru to Aulë in the matter of the Dwarves, she besought Eru (through Manwë) asking him to give life to things made of living things not stone, and that the Ents were either souls sent to inhabit trees, or else that slowly took the likeness of trees owing to their inborn love of trees. ~ Letter #247

Considering the infromation we get from the books I personally have to go with the second theory.

For more informatin I recommend the quite nicely done essay on Ents by Ardamir.

Before reading the essay I was myself quite sure that there were no more Entwives, though after doing so the idea of some Entwives having managed to escape Sauron's attack seems to me if not likely or probable, then at least possible.

[Gwathagor]

Interesting theory, Might, concerning the shape-changing of Ents. I suppose it's possible, but I would have thought them more, um, concrete and earthy than to go about actually changing their form. Certainly, they bore a superficial resemblance to trees, in certain of their characteristics (quality/quantity of beard, number of fingers, size, color, texture), but the resemblance didn't extend to their physical structure. Ents are closer to trolls than they are to trees, in that their figure is essentially mannish (two arms, two legs, head, face), and I, personally, don't imagine them using quite the kind of magic necessary to change that. Entish "magic" is more, well, natural and down-to-earth. It's like a speeding up of natural processes; the things they do that might be considered "magical" are things that happen anyway, only much, much faster. Shape-changing wouldn't be in their line of work, because it's unnatural.

EDIT: Well, Might, I just read the essay you linked to, and your theory makes a great deal more sense based on hints and suggestions in some of Tolkien's letters. I'm still skeptical, but it's definitely within the realm of possibility.

EDIT 2: I'm even less sure of myself now, after reading the part where Treebeard says: "sheep get like shepherds and get like sheep; but it is quicker and closer with Ents." It gives even MORE credence to The Might's theory, darn it. To what extent had you envisioned the Ents taking the shape of trees, Might? Would it be an literal physical change, as in the sprouting of branches and roots?

[The Might]

As this quote

A few [of the Ents at the Entmoot] seemed more or less related to Treebeard, and reminded them [Merry and Pippin] of beech-trees or oaks. But there were other kinds. Some recalled the chestnut: brown-skinned Ents with large splayfingered hands, and short thick legs. Some recalled the ash: tall straight grey Ents with many-fingered hands and long legs; some the fir (the tallest Ents), and others the birch, the rowan, and the linden.

shows they resembled the treeds, they reminded the Hobbits of the trees. Clearly there would be a difference between the Ents and the trees, but possibly this difference would diminish if the Ents grew treeish.

So it seems likely to me that it was due to their special assignment as shepherds and not as normal trees that they looked different, when they abandoned it they blended in with the others.
I know this sounds a bit strange, but as long as you have skin-changers capable of transforming into bears I don't see why Ents wouldn't be capable oftaking the shape of the trees they cared for.
Sounds quite like something Yavanna would think of.

[Gwathagor]

Yes, I suppose so. Though, it does say that the Ents recalled certain trees, which implies a fairly weak level of resemblance.

[Aaron]

I always found that to be tragic too, that the Ents, these noble and ancient creatures, were doomed to die out because they cannot find their wives. What makes it worse is that the Ents do not seem to want to look and I think that the Entwives probably didn't want to be found. Maybe they just moved to the Old Forest and slowly became more treeish. Only maintaining the tiniest sparks of life.
But I remember a while back that some guy declared that he had found the Entwives within Tolkiens work and the the author had used a variety of clues to conceal where they were. Whether this was all nonsense I'm not sure but it certainly intrigued me none the less.
Ah, here we are:

http://forum.barrowdowns.com/showthread.php?t=12512

[Eönwë]

Maybe they just grow like trees. Well, what I mean is they start slightly tree-ish, but as they get older develop into more tree-like beings, depending on which tree they chose to guard.

I can just imagine it: Over time. First a little dark patch, then a spot, then a lump, then a shoot, and after about 20 years of slow growing, a small brach.

edit: A sad thought just came into my mind. Maybe all ents are sufferers of a disease, like this?