> hmm... just got an idea... would it be possible to program it to so that uncooperative BBcodes (i.e. ones not written correctly) are marked out somehow? Say a shiny red (or some other color)? (in the submitted/previewed post?

Hmm, I could see that being useful when you're dealing with a lot of BBCode, like when building a [table] by hand:

    [table]
      [row] [cell]foo[/cell] [cell]bar[/cell] [cell]qux[/cell] [/row]
      [row] [cell]1[/cell] [cell]3[/cell] [cell]5[/clel] [/row]
      [row] [cell]2[/cell] [cell]4[/cell] [cell]6[/cell] [/row]
    [table]

(See the typo?)

Though that kind of polish tends to be pretty expensive in terms of opportunity cost.