Google, Motorola, and Palm all have large patent portfolios to defend themselves with. Motorola literally invented the cell phone. Palm has a ton of important patents from the Treo days. Google has been gearing up for fights with the likes of Microsoft and Amazon.
For example, Engadget ran an analysis on Palm vs. Apple a while ago. They found that both companies infringed heavily on each other's patents, for things from list scrolling to three-way calling. Starting that particular food-fight would probably end badly for both companies.
HTC is a much softer target. HTC is relatively new in the business, with a smaller patent portfolio. They're also a foreign corporation, which may give Apple some home advantage in the courtroom.
At its core, this is all about Google. Every single Google developer phone has been HTC, as well as the G1 and Nexus One. If Apple prevails, that would be a pretty big blow to Google just as Android is beginning to spread its wings.