We've initiated work on Arch Linux for the HP TouchPad and have made great progress over the past week. It's currently in a developer-preview state, but if you're willing to acquire some USB connection cables, you can join the fun.
Going forward, we plan to package up a nice bundle with everything you need to easily enjoy Arch Linux ARM on the TouchPad and have a truly open tablet computer. In the meantime, check out the following resources: