With the last three Rockbox builds I've tried, the menu button on my Sansa e280 no longer behaves as it did but instead does exactly the same thing as the power button.
I've observed this when using the database, where pressing the menu button used to give me a menu of whether to insert, queue or play the selection (and so on) but now jumps me back to the main menu.
This is absolutely crippling, and I'm about to report it as a bug but I thought I'd check that there isn't some other explanation that I haven't thought of.
I first observed this problem in r15751. I've since tried r15871 and r15909 but neither have corrected it. r15627 was the last working one I've used.
I'll try the latest build (I haven't yet, because I didn't see any mention of this in the changelogs) and if it doesn't work I'll try reverting to r15627 in the meantime, and to confirm that version does work.