I chatted a bit about <span> with Tim and others on IRC, and there's no general objection to it. My original objection was that at the time it was requested there was no way to set up style sheet classes through the wiki, making it a semantically void element no better than the hated <font>. <span style="font-face:'Arial'"> isn't any better in any legitimate sense than <font face="Arial">.
However since then we _have_ added a way to add to the global stylesheets in the wiki (at least for the main skins), so I withdraw this objection. It could be easier, clearer, and more cohesive than it is (doesn't apply to all skins etc), but it's a start.
<span> is now added to the whitelist for 1.4.
-- brion vibber (brion @ pobox.com)