I've been enjoying my 990/7125/LFM-1Plus combo for a few weeks now.
The only thing left for me to do is find rear surround speakers.
The problem is that they would have to be mounted on the ceiling.
This is because there is no back wall to mount speakers, there is a staircase that goes to the 2nd floor.

Can anyone recommend good quality speakers that are able to be mounted to the ceiling?
I'm somewhat worried about the speakers falling if they were not designed to be ceiling mountable.
All my speakers are B&W (Front DM603, Center CC6 S2, Surrounds DS6).

I investigated this a couple years back (side wall mounting problems) and found that the biggest problem was finding brackets that I'd trust. The load on a wall bracket is one thing - but those same brackets were not rated for ceiling mounting and the attachment methods didn't lend themselves to being supported from a ceiling. If you could screw into a joist, it'd be a possibility, but even then the folks I talked to (mainly at Paradigm) seemed skittish about screws being able to carry the weight in that orientation. The ceiling brackets I just turned up with a quick search seem to max out at anywhere from 5 to 11 pounds - for comparison, my Paradigm ADP's weigh 22 pounds each and my Axiom M3ti's weigh 13 pounds each.

You could probably come up with something that would carry additional weight if you got a bit creative, but I ended up moving my ADP's to surround back and getting a little creative with my M3ti's and Axiom's FMB ( full metal bracket ).

Since you already own B&W speakers I would highly suggest you give consideration to the "Custom Installation Series" by B&W.

http://www.bwspeakers.com/index.cfm/fuseaction/products.ranges/label/Range%20In%20wall%20Series

They have a wide variety of speakers specifically designed for ceiling and wall mount applications. Prices and quality range from entry-level to high-end.

Hands down, the best ceiling in walls are the Polk CSi 80 speakers. They can be found on the Polkaudio.com site.

Haropa makes some pretty heavy duty speaker mounts. I don't know if this will help but it can't hurt to look.

http://www.haropa.com/index.html

Those speakers have to be placed inside a wall or ceiling. There is no way that I can do that since the ceiling is the poured concrete floor of the master bedroom upstairs.

I wasn't sure if something like this would work:
http://www.bwspeakers.com/index.cfm/fuseaction/products.models/label/Model%20M%2D1

I misunderstood. I did not realize your ceiling was a concrete slab. I see your dilemma now. At least part of your solution may need to incorporate concrete anchors or concrete tapcon screws in conjunction with a standard mounting bracket (i.e. replace the original screws provided with a bracket with concrete screws). As long as you don't exceed the weight limit for any given bracket supplied with the speakers this may be a viable solution. The light to medium duty tapcon shear and pull out values in concrete are pretty stout http://www.confast.com/products/tapcon-concretescrew.aspx
I think you can pick these up at Lowe’s or Depot as well.

I did come across an inexpensive ceiling bracket rated at 20 Lbs from omnimount that may be of interest to you. http://www.omnimount.com/consumer/

We've used these to hang 40 LBS light fixtures.
http://www.hilti.com/holcom/modules/prcat/prca_navigation.jsp?OID=-9769
Jim