its because the more recent LIRC versions have changed the key definitions. For it to work with XBMC, for those keys, LIRC needs to report BACK and OK to XBMC. So you should see BACK and OK from irw.
You have three choices.
1. Create a modded Lircmap.xml in your .xbmc/userdata (not sure I got name right), changing under mceusb
<select>OK</select> to <select>KEY_OK</select> and
<back>Back</back> to <back>KEY_BACK</back>
You can find the default xbmc Lircmap.xml file in /usr/share/xbmc/system if I recall correctly.
2. Modify LIRC file /etc/lirc/lircd.conf I believe it is.
By default LIRC will be configured to include a file from the path and file indicated in that file. copy that file to /etc/lirc, modify it, rename it lircd.conf and change KEY_OK to OK and KEY_BACK to BACK. (I might have some of the file names wrong, I'm at a win7 system right now without easy access to a linux system).
3. Dump LIRC all together. With more recent kernels it is possible to run MCE remotes without LIRC by using the drivers builtin to the kernal. There's a bit of work involved and I'll be posting a how-to when I have a chance.
Now that said, virtually all the keys in LIRC have been changed to KEY_something. So if you were purely using LIRC, the MCE remote shouldn't work at all. So I think that you both LIRC and the kernel driver working (in possible conflict) right now. I believe that XBMC is currently getting the keys from the kernel through SDL not from LIRC. If this is the case (1) and
(2) will likely lead to double key presses, one from LIRC and one from the kernel for every remote button press. I believe this to be because when I started reconfiguring my system without LIRC, many of the MCE remote keys worked, but OK and BACK did not. So likely you need to either you need to disable the kernel driver for (1) and (2) (google that), or go with (3) when I post the how-to. There are some posts on option (3), but I found I had to do more to get it fully working.
EDIT: Take a look at this post
http://forum.xbmc.org/showthread.php?tid=112649 for (1), but still think you end up with double key presses for some keys and still need to blacklist the kernel MCEUSB driver to avoid that, or go with option (3)
