palm didn't do anything that any other developer can't do. You remember all the press releases about how easy it is for people to code for wm5 because of all the hardware APIs. Anyone can write a today plugin to dial the phone. Or even a today plugin to display video from the camera.. all the APIs are there.
Palm is just the first device maker to actually do this. The most hp has ever done is buy some third party app, rename it and pack it into the rom. This is basically the same thing but palm wrote the software themselves. Plus i think they added a lot more than just a task swithcer.
Even the new hps have their own today meter plugin. WM5 just allows for more/easier direct hardware access though standard APIs.