Both the current Momentus XT and Toshiba MQ01ABDH Hybrid are too tall. The WD Hybrid is a 500 GB 5mm HDD, but it was just announced in September(See announcement
here). Seagate also teased a 7mm drive in 2011(
here), but I don't recall if anything came out of it. So...
As for SSDs, you have a bit more options in 7 mm. Even more if you open up the SSD metal/plastic case and just use the "guts".
What notebook and what will you need the space for? First make sure it is actually a 7mm bay and not an issues with rubber rails(such as in the case of the Lenovo W520).
As for reliability, Samsung 830 and Plextor M3P are both 7mm and are usually regarded as reliable, due to selection of parts, warranty, or whether the components are made in-house.
Samsung SSD 830 Review (256GB)
Plextor PX-M3P SSD Review
If reliability is really paramount, you could also try an Enterpise SSD. They may be better tested and offer better firmware. However, they may not be compatible(you will have to make sure it is a SATA SSD, limited capacity(some top off at 400GB), and just may not be worth it in the long run depending on your needs.
Ideally, a backup solution is needed regardless of what drive you choose.