A few things...
A couple new things:
-First off, I want to retract anything positive I've said about the momentus XT in regards to speed. After about the first month it slows down to feeling like a regular drive. Despite updating to the latest firmware, its speed issues persist, and it has been known to have issues with MBP's. I'm going to try to return it and buy a 1TB 9.5mm drive instead.
-So while swapping out the 500GB Momentus XT that I had in my OWC optibay for the 500GB WD black I'm running now, I decided to try a 12.5mm drive in the optibay slot. I took my 1TB 12.5mm Samsung drive which I use as backup and installed it in the optibay, and put on the bottom panel. And it fit really pretty good! The screws went in nicely, they pulled the panel inward more than normal, but that's to be expected with the thicker drive standing out a little bit. The screw on the side (right next to the drives) went in with, predictably, the most difficultly, but it wasn't so bad. In the end there was a nearly imperceptible bulge, if I can call it that, that was significantly less than 1mm. I tested to see if the computer would wobble on a flat surface and it didn't. Now, it might not be good for the lifetime of the drive to have it making direct contact with the bottom panel, but the setup seemed entirely run-able.
This was on a 2010 13" uMBP with the OWC DataDoubler.
Of course, right now there is no storage advantage to be won by running 12.5mm drives, as 1TB drives are available in 9.5mm, and 1.5TB drives in 12.5mm aren't available yet.
I just wanted to share my findings, if someone needs more storage space and has a 12.5mm 1TB on hand, I think this setup works well.
-On the note of 1.5TB 12.5mm drives, I did a little looking into this. I've recently upgraded my entire music library to lossless, started upgrading my movie library to ~8GB 1080p movies, and gotten into the iOS 5 betas of which I've been storing several disk images. Anyway, so I could seriously use more space. Running a 1TB 9.5mm in the optibay, and a 1.5TB 12.5mm in the main bay would be ideal, but 1.5TB 12.5mm is not to be found. Well, I heard some conflicting reports on whether Seagate's external 1.5TB goflex was 12.5mm or thicker in height. It seemed that the drives originally launched with 14 or 15mm thick, 4 platter, 375GB/platter, drives, but that in recent months, with 500GB platters becoming available (such as in 9.5mm 2 platter 1TB drives), that seagate had begun using 3 platter, 12.5mm drives in the goflex drive. Anyway, so I called Seagate and they won't divulge the information. I even told them that I would have no use for the drive if it was thicker, and asked if I could buy a drive, see how thick it was, and then return it if it was thicker than 12.5mm, but they didn't go for that

Now, I think its possible that one could open the drive pretty invisibly, check, and still be able to return it as new if its too thick. I might just do that, but its kind of sketchy, and all on the rumor of 12.5mm drive lurking in there.
Iomega however has a 1.5TB 2.5" external drive available (
http://www.newegg.com/Product/Produ...rd+Drives+-+External-_-Iomega+Corp-_-22186316) which on newegg is listed as being 0.55 inches thick. This converts to 14mm exactly, which, subtracting a 12.5mm drive, would leave 1.5mm thickness between both the top and bottom parts of the case. 3/4 mm thick plastic for the case is already very thin, iomega itself lists the same dimensions, so it seems highly likely that this drive is using a 12.5mm drive and not a taller one. Even the 14mm thickness of the drive + enclosure is the next thickness up that most bare drives would have if they are thicker than 12.5mm. Still it's a $220 drive, and even if it is 12.5mm, it could have a soldered on USB board as some companies have begun to do. I should mention that on amazon and on some smaller sites it is listed as 0.6 inches, or 15.25mm, but amazon UK, newegg, B&Hphoto and go.iomega all list the drive as 0.55 inches. Not sure which is correct. I will give iomega a call on Monday and see if they are more willing to reveal the drive inside than seagate is. Here's hoping
Here's the link to that drive on iomega's site:
http://go.iomega.com/en/products/ex...estige-usb-3/?partner=4735#tech_specsItem_tab
I just get really excited by the idea that if a 12.5mm 1.5TB drive is available, it is possible, with the squeezed in 12.5mm in the optibay, to run
3TB in a 13" laptop!
Also, here are the photos of the 12.5mm drive in the OWC optibay: