"The best idea ever for a smartphone" could be a lot of things, but we'd never guess that it'd be a portable external monitor. That's exactly what GeChic's 13.3-inch On-Lap 1302 monitor is being touted as, however. The USB-powered monitor has a 1366 x 768 display that supports HDMI and VGA inputs with included adapters for its proprietary 30-pin connector, and there's also an optional Mini DisplayPort dongle if you need it. GeChic says it's designed for laptops, smartphones, cameras, and game consoles.
If the monitor's advertised 16ms response time and 45-degree viewing angles don't scare you, the set up to get it connected to your smartphone definitely will. To work with an iPhone 4, 4S, or iPad you'll need Apple's $39 Digital AV Adapter and, unless you've got a nearby laptop to power the screen, you're going to want a 5V USB battery back, too. If you want to get some Plants vs. Zombies from your Android phone onto the On-Lap, you'll need to make sure your phone supports MHL (Mobile High-definition Link), like the Galaxy S II, and you'll need an MHL to HDMI adapter. Oh, and the On-Lap 1302 doesn't have a built-in speaker, so you'll probably want to hook up your phone to something like the Jawbone Jambox, too.
As you can imagine, it all becomes quite a mess of cables pretty quickly, but it's still the only such portable monitor we've seen that you can plug a smartphone into — GeChic's On-Lap 1301 was designed for laptops only, and Toshiba's funky Mobile LCD Monitor connects via DisplayLink, which doesn't play nice with smartphones. If you're interested, see our product database for the On-Lap 1302's full specs, and see the video below for some more information on the $229.99 On-Lap 1302 as well as some "guest appearances" by Jeremy Lin, Spongebob, and Patrick.