Sanwa has an HDMI adapter on sale in Japan for $73, and if you want something more readily available, the Chinese NOOSY has already crept onto American soil, with a price tag of $69.99. NOOSY's contraption does not support movies purchased from iTunes, or protected with Apple's Fairplay scheme, and it also doesn't output your Safari screen (same with Sanwa, we guess). However, the Chinese adapter also supports iPod touch 4G, whereas the Japanese gizmo works only with the iPhone 4 and the iPad. There are other non-official solutions out there, such as the Wolverine module, sold for $59.99 on Amazon, and the main complaint is that they don't charge the battery, so one or two movies, and you are done.
We know, we know, we'd rather prefer wireless streaming too, but that's surely not going to cost you sixty or seventy bucks, so you'd better get one of those thin HDMI cables under the carpet, and call it a day.
source: NOOSY & Akihabara