Apple has filed the patent lawsuit in the US District Court in Delaware, as well as the US International Trade Commission. Apple claims that HTC infringes upon 20 of its patents related to the iPhone's user interface, as well as the underlying architecture and hardware (multitouch?).
"We can sit by and watch competitors steal our patented inventions or we can do something about it," Apple's CEO Steve Jobs said in the statement, "We've decided to do something about it. We think competition is healthy, but competitors should create their own original technology, not steal ours."
HTC has apparently been caught off-guard - it hasn't even been served yet. "We only learned of Apple's actions based on your stories and Apple's press release," the Taiwanese company states, "We have not been served yet so we are in no position to comment on the claims. We respect and value patent rights but we are committed to defending our own innovations. We have been innovating and patenting our own technology for 13 years."
Engadget has already drilled down into Apple's filing, and has a rundown of the patents involved in this case - contrary to Apple's PR, only 10 patents seem to be listed, instead of 20. Some of the patents, as John Gruber points out, are of the typical software patent nonsense variety, such as "Unlocking A Device By Performing Gestures On An Unlock Image", while others are more hardware-related and therefore much harder to judge.
The filing also details which HTC products are involved: the Nexus One, Touch Pro, Touch Diamond, Touch Pro2, Tilt II, Pure, Imagio, Dream / G1, myTouch 3G, Hero, HD2, and Droid Eris - pretty much everything.
The interesting detail, as Engadget points out, is that the Android phones were specifically mentioned for their software, whereas the Windows Mobile devices were mentioned because of their hardware. It seems like Apple wants to avoid angering Microsoft, but has no qualms about dealing with Google. Interesting.