WP Custom Meta DropDowns: A dropdown function for custom meta that mirrors wp_dropdown_categories. To use it, set up a form that has a "get" method, pass in your custom meta data to check against, and then use an added query_var and pre_get_posts in order to pass the new meta query in to $wp_query.

custom_meta_dropdown_backend.php

<?php
add_filter( 'query_vars', 'custom_meta_dropdowns_query_args' );
function custom_meta_dropdowns_query_args($vars) {
	$vars[] = 'locations';
	$vars[] = 'area-of-practice';
	return $vars;
}

add_action( 'pre_get_posts', 'set_physicians_args' );
function set_physicians_args($query) {
	if ( $query->is_main_query() && $query->is_post_type_archive('physician') && !is_admin() ) {

		$location = get_query_var('locations');
		$practice_area = get_query_var('area-of-practice');

		if ( $location != -1 || $practice_area != -1 ) {
			$meta_query = $query->get('meta_query');

			if ( $location != -1 && $location != '' ) {
				$meta_query[] = array(
					'key' => 'physician_location'
				,	'value' => $location
				);
			}

			if ( $practice_area != -1 && $practice_area != '' ) {
				$meta_query[] = array(
					'key' => 'physician_practice'
				,	'value' => $practice_area
				,	'compare' => 'LIKE' // If the item is in a serialized array, use LIKE.
				);
			}

			if ( !empty($meta_query) ) {
				$query->set( 'meta_query', $meta_query );
			}
		}
	}
}

custom_meta_dropdown_frontend.php

<?php
if ( is_post_type_archive('physician') ) {
	$l_items = get_posts('post_type=location&numberposts=-1&orderby=menu_order&order=ASC');
	$location_args = array(
		'name' => 'locations'
	,	'items' => $l_items
	,	'show_option_all' => '-- All Locations --'
	,	'selected' => get_query_var('locations')
	);

	$a_items = get_posts('post_type=area-of-practice&numberposts=-1&orderby=title&order=ASC');
	$areas_args = array(
		'name' => 'area-of-practice'
	,	'items' => $a_items
	,	'show_option_all' => '-- All Practice Areas --'
	,	'selected' => get_query_var('area-of-practice')
	);
?>
	<div class="physicians-filter">
		<h3 class="block-title">Filter Physicians</h3>
		<form method="get" action="/physicians">
			<?php custom_meta_dropdowns($location_args); ?>
			<?php custom_meta_dropdowns($areas_args); ?>
			<button type="submit">Filter</button>
		</form>
	</div>
<?php } ?>

custom_meta_dropdowns.php

function custom_meta_dropdowns($args) {

		$defaults = array(
			'name' => ''
		,	'value' => 'ID'
		,	'items' => array()
		,	'echo' => true
		,	'show_option_all' => ''
		,	'selected' => ''
		);

		$vars = wp_parse_args($args, $defaults);

		$select_o = '<select id="'. $vars['name'] .'" name="'. $vars['name'] .'">';
		$select_c = '</select>';

		if ( $vars['value'] == 'slug' ) {
			$item_key = 'post_name';
		}
		else {
			$item_key = 'ID';
		}

		foreach ($vars['items'] as $key => $item) {
			if ( !is_object($item) ) {
				continue; // We can build this in later to accept IDs as well as objects, if necessary.
			}
			$value = $item->$item_key;
			$title = $item->post_title;
			$selected = '';
			if ( $vars['selected'] == $value ) {
				$selected = 'selected="selected"';
			}
			$options[] = '<option value="'. $value .'" '. $selected .'>'. $title .'</option>';
		}

		if ( empty($options) ) {
			return '<!-- No options available -->';
		}

		if ( $vars['show_option_all'] !== '' ) {
			array_unshift($options, '<option value="-1">'. $vars['show_option_all'] .'</option>');
		}

		$output = $select_o . implode("", $options) . $select_c;

		if ( $vars['echo'] == true ) {
			echo $output;
		}
		else {
			return $output;
		}
	}