Well, ippa has convinced me that
:mouse1 etc. are just a bit too cryptic, and people would need to look at the docs if they forget which number means which button. 4 and 5 are especially confusing; even I have a hard time remembering which one is wheel-up, and which is wheel-down. I also realized that the 'advantage' of allowing an arbitrary number of mouse buttons is rather tenuous, given that the vast majority of mice fit the 3-button-plus-wheel (or less) schema.
So, I've changed over to
:mouse_wheel_down. Other mouse buttons, if they should occur, will generate
:mouse_N symbols (where N is the button number). No reason to exclude them altogether (although I suspect the current SDL doesn't even recognize buttons above 5).
Another topic that came up in the comments yesterday was compatibility with Ruby 1.9. It bears repeating here (with a lovely diagram for emphasis), that it would be helpful for the Ruby 1.9-eager folks to submit a Rubygame bug report for anything in Rubygame (either svn trunk or the latest release) that doesn't work with Ruby 1.9 (including/especially compile errors). Please be sure to select the "Ruby 1.9" group, as illustrated below: