Coming from using google reader mostly on my phone, I too miss the next button. One thing I found with selfoss is that is isn't necessary to hit the 'x' to close an article, tapping anywhere that isnt a link will bring you back to where you were on the list.

I tinkered with something along those lines yesterday...

I added a "Throw" button (i.e. mark as read and go to the next message) to the smartphone view (and removed all the google/twitter/etc stuff as I don't use it) --- not exactly what you want but it might be useful to some people...

I just created a patch and posted it here: http://pastebin.com/U660B1fF
Save it as a file and run "patch -p1 < fileUSavedItAs"

(although naturally this requires the patch utility)

Disclaimer: I probably don't adhere to the style etc and I haven't tested it excessively, but it works on my phone :)

Edit: Oh, and as with an update you should delete (at least) public/all.js and clear your browser cache...

I'd like to second that idea. Swipe gestures to go to the next/previous items might even be better, but I'm not sure how simple it is to do that in html5.