Here’s a pretty good real-life example that shows why I think it’s a bad idea for Android’s Ice Cream Sandwich to move their navigation buttons to software (and not hardware).

From Danny Sullivan’s review of the Nexus Galaxy / Prime:

Where it all goes wrong is with the space bar. It sits right above the system’s home key:

I find when I’m really on the move skating, I’m always hitting that home key by mistake rather than the space bar and so exit out of what I’m typing. This never happens with the iPhone or the Droid Charge, because they use “hard” menu buttons that you have to physically depress. You can’t accidentally push them.