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A sorceror can wield it with no penalties whatsoever.
If you wield a shield with which you aren't proficient, you take the armor check penalty of the shield to most actions (including attack rolls, most movement-related skill checks, etc). Mithral bucklers have no armor check penalty, and therefore have no penalty for nonproficiency.
The sparkcaster doesn't even need Shield Proficiency, as the penalty for non-proficiency (ACP applied to many skills, rather than just a smallish set) is 0, as the ACP of a Mithral Buckler is 0.
Ah yes, the old mithril buckler. The only one who can't use it is the monk! (and now assassin, and anyone using a monk's belt). It's an awesome thing, and I'm pretty sure that the only reason it's not included in any standard equipment list is so that it won't be widely noticed.