the (free, open) Android version is more akin to a rootkit
Carrier IQ is not free or open. The post you responded to was arguing that closed source is more difficult to analyse, which is clearly true. If Carrier IQ were open source, we would have known about it years ago, and we wouldn't need to reverse engineer it to figure out what, when and how it's doing what it does, and under what conditions the logs get transferred to remote servers, etc.
I would also argue that, as much as we dislike Carrier IQ, it isn't really a rootkit - the software itself makes no effort to hide its presence, which is one of the defining characteristics of a rootkit. Also, you say that the Android version has a "backdoor" - could you provide a reference for this? As far as I can see, this is not actually true, as it doesn't enable any secret authentication-bypassing remote access (which would be the very definition of a backdoor).
You're right and though the discussion was leaning that way, I didn't actually mean to take a position on open versus closed. No, the software in question doesn't technically meet the definition of a rootkit but I maintain that it's "akin" to one. It is not part of Android as released by Google, and although it doesn't alter APIs to hide itself (such as removing itself from process lists or filesystem calls), it's not an application that shows-up in the launcher, nor do users have any meaningful control over it. A backdoor provides a means for bypassing access control... and this software, as it's been seen on many Android devices, is a secret means of accessing data stored on or passed by (even over SSL) potentially PIN-secured, filesystem-encryped devices. It doesn't seem to be remotely initiated so maybe it's not a backdoor so much as a back window. They can't come in but they can stand outside and see everything you do.