I just tried IE tab, and alt+s/p tries to post, as well.
Right - Here's the source of the confusion - if you look carefully at the Reply screen, at the bottom, there are shortcuts posted - alt+s, alt+p, to post or preview, respectively.
In my post about placing the cursor in the Reply screen, Carbon said that it would be nice if the above shortcuts were also fixed. Calum said that the shortcts don't work in FF but did in IE. I then determined that adding the 'shift' key allowed the "save/preview' shortcuts to work in FF.
The cursor placement hasn't been worked out yet, other than the Tab-Tab, which Calum mentioned.
I was curious as to why the shift+alt+p/s didn't 'work' for you when it did for Carbon and me - now I know!