I have had some clients mount these on the walls with no issue. You just have to verify you still have adequate coverage especially if you gave surveyed with the AP's positioned up by the ceiling. You can check the antenna pattern on this guide.
We have wall mounted these, but only when using a simple "L" bracket that
will orient the AP horizontally Here is the Cisco info in relation to wall mounting
Access points with integrated antennas perform best when the access point is mounted on horizontal surfaces such as a table top or ceiling. For advanced features such as voice, location, and rogue access point detection, ceiling mounting is strongly recommended. However, for smaller areas such as conference rooms, kiosks, transportation environments, or hot-spot usage where data coverage is the primary concern, the unit may be wall mounted using wall anchors or screws.
I can confirm. I have a few APs mounted to the wall due to ceiling structure. As mentioned, just make sure your coverage is fine mounted in this fashion and you should be good. Ive even used the regular mount (which is included with the ap ) and use the drywall pieces to attach it to the wall.
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
You might get better performance with the adjustable rubber ducky antennas instead of the little caterpillar feet antennas (I know, such great technical terms)
Anyhow, I've seen installation pictures (that I can't find) with the 3502 mounted on the wall basically with the top antenna up, bottom antenna down, and middle antenna out to the side..... (I think each radios antennas are on the sides instead of top to the bottom, assuming you have the Cisco label in the correct direction)
Of course, if you rotate your AP sideways so that all antennas for the radio are Top side and bottom side, then you just point them same direction.... It would make more since if I had the picture though....
In the end, go with whatever helps your coverage....
We had a case where we were going to install 30+ WAPs to a school. The installers were "reputable Wi-Fi installers". Then one of the wall-mounted WAPs was giving wierd signals. When I looked at it, the "genius" mounted the WAP on the A/C and vents. we not only had them take it down and mount it correctly the installer had to pay for the damage done to the vents.