Yes, Ebay has the replacement back for $20 and free shipping. You'll need to remove the outer housing and disassemble the inside. It will replace the kickstand and camera lens cover. I would then proceed to consider modifying the lens cover from the damaged portion with a better quality glass, though that seems unlikely (and probably more expensive than it's worth,) considering how exact you would have to make it, and the trouble finding a source that could properly construct it at a reasonable cost.
$20 is an easy solution.
PS: It's fairly possible that, considering how weak the lens cover (because it's really not an actual lens, rather it's like calling a polarizer filter a lens when it's just a piece of glass that protects or filters various lighting elements, with absolutely no effect on the form of the subject shape or size,) that people are actually bumping into stuff while it's in their pocket, and they don't realize it. It also seems to be possible that those that have blurred images may actually have damaged the camera lens itself, as otherwise the cracked lens cover doesn't seem to affect the image quality. I could be completely wrong as to whether the camera lens cover acts as a lens that is necessary to focus the image, if in fact the camera gets blurry without it. To prevent further damage though, I would definitely want to preserve the camera lens (it works as well as my Konica-Minolta Dimage 200 (though without the telephoto. . . Don't confuse digital zoom with optical as they are totally different things.)
Are people suggesting that temperature conditions caused it to crack? That the large metal ring somehow contracted and expanded, causing the plastic---which should be flexible, though maybe it does--crack?
I would be curious to try the micro and macro lens adapters though certainly will pass on gluing them to the phone. . .
And for those calling it a $200 camera. . . It most certainly isn't, even if it's what you paid for it. Normally something like this costs much more to produce, and whatever service provider (Sprint?) ends up paying HTC the difference in what you bought it for, because they want you to sign a contract which gives them more than $1500 over the next two years.
Same with game systems. . . You buy something that is often produced at a higher cost at the beginning of production sold to you cheaply, and it's assumed you will buy games over the next several years that will give game producer's royalties to the system designer, as well as whatever else in accessories bought. It's not a cheap phone though I do question their cheap lens cover. . . It should have been as durable as the front glass, and it clearly isn't. . .