Well it finally happened, Apple removed the HyperCard sub-directory from the main Apple site and you can no longer buy HyperCard from the Apple store. Of course, HyperCard was never updated for Mac OS X and I'm pretty sure the last update to HyperCard was version 2.4.1 back in 1998. But the removal of the product from the Apple site is the final nail in the coffin.
If you want to upgrade your old HyperCard stacks and use an xTalk-style language, then your best bet is probably Runtime Revolution. If you want more info on HyperCard, then you might want to check out the HyperCard wiki page.
Meanwhile, the Open Source PythonCard project will continue to plug away at a worthy successor using the Python programming language that will run on Windows, Linux, and Mac OS X. We're finally going to make a 1.0 release later this spring and I plan to get back to work on a more integrated HyperCard-like environment for PythonCard. If you would like to help make that happen, I invite you to email me and join the project.
Rest In Peace HyperCard. You are not forgotten and your children live on...