First pass. It's not pretty yet, but it works.

columns.css

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+.column-grid {
+	clear: both;
+	}
+.column-grid:after {
+	content: ".";
+	display: block;
+	height: 0;
+	clear: both;
+	visibility: hidden;
+	}
+.column {
+	float: left;
+	margin-right: 5%;
+	}
+.column-first {
+	margin-left: 0;
+	}
+.column-last {
+	float: right;
+	margin-right: 0;
+	}
+
+
+/**
+ * Formula for getting the width of each column.
+ *	$width = ( 100 + $margin-right ) / ( $grid ) * ( $span ) - ( $margin_right )
+ *	$x     = ( 100 + $margin_right ) / $grid
+ */
+
+/* $x = 6.5625 */
+.column-grid-16 .column-span-16 { width: 100%;     }
+.column-grid-16 .column-span-15 { width: 93.4375%; }
+.column-grid-16 .column-span-14 { width: 86.875%;  }
+.column-grid-16 .column-span-13 { width: 80.3125%; }
+.column-grid-16 .column-span-12 { width: 73.75%;   }
+.column-grid-16 .column-span-11 { width: 67.1875%; }
+.column-grid-16 .column-span-10 { width: 60.625%;  }
+.column-grid-16 .column-span-9  { width: 54.0625%; }
+.column-grid-16 .column-span-8  { width: 47.5%;    }
+.column-grid-16 .column-span-7  { width: 40.9375%; }
+.column-grid-16 .column-span-6  { width: 34.375%;  }
+.column-grid-16 .column-span-5  { width: 27.8125%; }
+.column-grid-16 .column-span-4  { width: 21.25%;   }
+.column-grid-16 .column-span-3  { width: 14.6875%; }
+.column-grid-16 .column-span-2  { width: 8.125%;   }
+.column-grid-16 .column-span-1  { width: 1.5625%;  }
+
+/* $x = 8.75 */
+.column-grid-12 .column-span-12 { width: 100%;   }
+.column-grid-12 .column-span-11 { width: 91.25%; }
+.column-grid-12 .column-span-10 { width: 82.5%;  }
+.column-grid-12 .column-span-9  { width: 73.75%; }
+.column-grid-12 .column-span-8  { width: 65%;    }
+.column-grid-12 .column-span-7  { width: 56.25%; }
+.column-grid-12 .column-span-6  { width: 47.5%;  }
+.column-grid-12 .column-span-5  { width: 38.75%; }
+.column-grid-12 .column-span-4  { width: 30%;    }
+.column-grid-12 .column-span-3  { width: 21.25%; }
+.column-grid-12 .column-span-2  { width: 12.5%;  }
+.column-grid-12 .column-span-1  { width: 3.75%;  }
+
+/* $x = 10.5 */
+.column-grid-10 .column-span-10 { width: 100%;  }
+.column-grid-10 .column-span-9  { width: 89.5%; }
+.column-grid-10 .column-span-8  { width: 79%;   }
+.column-grid-10 .column-span-7  { width: 68.5%; }
+.column-grid-10 .column-span-6  { width: 58%;   }
+.column-grid-10 .column-span-5  { width: 47.5%; }
+.column-grid-10 .column-span-4  { width: 37%;   }
+.column-grid-10 .column-span-3  { width: 26.5%; }
+.column-grid-10 .column-span-2  { width: 16%;   }
+.column-grid-10 .column-span-1  { width: 5.5%;  }

columns.php

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+<?php
+/*
+ * Plugin Name: Columns
+ * Plugin URI: http://justintadlock.com
+ * Description: A [column] shortcode plugin.
+ * Version: 0.1 Alpha
+ * Author: Justin Tadlock
+ * Author URI: http://justintadlock.com
+*/
+
+
+add_action( 'init', 'columns_register_shortcodes' );
+
+add_action( 'wp_enqueue_scripts', 'columns_enqueue_styles', 1 );
+
+function columns_enqueue_styles() {
+
+	wp_enqueue_style(
+		'columns',
+		trailingslashit( plugin_dir_url( __FILE__ ) ) . 'columns.css',
+		null,
+		'20120913',
+		'all'
+	);
+}
+
+function columns_register_shortcodes() {
+	add_shortcode( 'column', 'columns_do_shortcode' );
+}
+
+function columns_do_shortcode( $attr, $content = null ) {
+	global $_columns_grid, $_columns_span;
+
+	if ( is_null( $content ) )
+		return $content;
+
+	$defaults = array(
+		'grid' => 10,
+		'span' => '', // will be overwritten to be the same as $grid if empty
+		'class' => '',
+	);
+
+	$attr = shortcode_atts( $defaults, $attr );
+
+	$output = '';
+	$classes = array();
+
+	$classes[] = 'column';
+	$classes[] = "column-span-{$attr['span']}";
+
+	if ( isset( $attr['class'] ) )
+		$classes[] = $attr['class'];
+
+	$is_first_column = false;
+	$is_last_column = false;
+	$allowed_grid = array( 10, 12, 16 );
+	$allowed_span = array( 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 );
+	$attr['grid'] = absint( $attr['grid'] );
+	$attr['span'] = absint( $attr['span'] );
+
+	if ( 16 == $attr['grid'] )
+		$allowed_span = array_merge( $allowed_span, array( 13, 14, 15, 16 ) );
+
+
+	if ( empty( $_columns_grid ) ) {
+		$_columns_grid = $grid = in_array( $attr['grid'], $allowed_grid ) ? $attr['grid'] : 12;
+		$classes[] = 'column-first';
+		$is_first_column = true;
+	}
+
+	if ( empty( $attr['span'] ) )
+		$attr['span'] = $grid;
+
+	$_columns_span = in_array( $attr['span'], $allowed_span ) ? $_columns_span + $attr['span'] : $_columns_span;
+
+	if ( $_columns_grid == $_columns_span ) {
+		$classes[] = 'column-last';
+		$is_last_column = true;
+
+		$_columns_grid = 0;
+		$_columns_span = 0;
+	}
+
+	$class = join( ' ', array_map( 'sanitize_html_class', array_unique( $classes ) ) );
+
+	if ( $is_first_column )
+		$output .= '<div class="column-grid ' . sanitize_html_class( "column-grid-{$attr['grid']}" ) . '">';
+
+	$output .= '<div class="' . $class . '">' . wpautop( do_shortcode( $content ) ) . '</div>';
+
+	if ( $is_last_column )
+		$output .= '</div>';
+
+	return $output;
+}
+
+
+
+
+
+
+?>