[blet]

I just found OnMyCommand (contextual menu plugin), which I used to add this function to OmniWeb.
Here's what I added to the OnMyCommand prefs so I could search google for highlighted text (assumes omniweb is default browser).

<key>COMMAND_LIST</key>
	<array>
		<dict>
			<key>ACTIVATION_MODE</key>
			<string>act_selected_or_clipboard_text</string>
			<key>COMMAND</key>
			<array>
				<string>open "http://www.google.com/search?q=</string>
				<string>__OBJ_TEXT__</string>
				<string>"</string>
			</array>
			<key>ESCAPE_SPECIAL_CHARS</key>
			<string>esc_with_percent</string>
			<key>EXECUTION_MODE</key>
			<string>exe_silent_popen</string>
			<key>NAME</key>
			<string>Google</string>
			<key>SUBMENU_NAME</key>
			<string>..</string>	
		</dict>
	</array>

The advantage of this over an in-browser method is that you can search from pretty much any app that lets you highlight text. Only thing that bugs me is searches appear in foreground rather than background tabs.